Doom Slayer

"Rip... and... tear!"

- The Slayer, after defeating a fellow Sentinel initiate in the Blood Arena

"No."

- The Slayer, after being asked if he has anything to say before killing the Dark Lord

The Doom Slayer is the protagonist and playable character of Doom, Doom II: Hell on Earth, Doom 64, Doom (2016), and Doom Eternal. He has also guest starred in the Quake games such as Quake Champions and Quake III Arena.

Identity
The demons first gave him the appellation "Doom Slayer" and also refer to him as the Unchained Predator, the Beast, and the Hellwalker. The UAC refers to the him as DOOM Marine, or The Slayer.

Before the character had any epithets in the story, he was most commonly called Doomguy in the fan community. This nickname is mentioned in Doom Eternal and used in his dialogue subtitles.

He is intended to be the latest iteration and continuation of the classic DOOM Marine who is the protagonist for most of the series. According to Tom Hall and John Romero (who worked on the original Doom), the original iteration of the DOOM Marine was the grandson of Commander Keen and the great-great-grandson of William J. Blazkowicz. Some fan-theories say the Doom Slayer's name could be '''William Joseph "B.J." Blazkowicz III but according to official interviews the developers clearly stated he has no name.''' The Doom Slayer's Blazkowicz ancestry may or may not be canonical for the modern iteration.

The developers have originally stated that the protagonist has no canon name because he is a representation of the player.

When asked directly on Twitter, John Romero replied humorously that his name really is Doom Guy (with a space, in two words - first word for name and second for surname).

Classic Doom (Doom, Doom II, and Doom 64)
The Doom Marine was originally a human from Earth who served in the Marines and was dishonorably discharged due to assaulting his commanding officer after refusing to fire upon a crowd of unarmed civilians, and was later transferred to the UAC Phobos base as security. The Marine soon found himself trying to prevent the demons of Hell from invading Earth. After having fought demonic hordes through Phobos, Deimos and Hell, he returned home to find Earth overrun by demons and his wife, child, and pet rabbit Daisy, among the billions of people killed, giving him further reason to pursue hunting the demons. He continued fighting the horde on Earth, before taking the fight to Hell once more.

Following this, the Marine was decommissioned. Some years later though, he was called upon again to fight the hellish force attacking a space station, prompting him to return to Hell where he defeated the Mother of All Demons. In order to save Earth, he decided to close the portal from inside Hell and continue the fight against the demons.

The Mother's sister, Resurrector, wanted vengeance, thus teleported him to Earth to kill him there. But he fought his way to Hell again, and defeated her there. Once more, chose to remain in Hell to fend off the demons.

Before Doom (2016)
At some point, he somehow became stranded on the world of Argent D'Nur, ending up in a different dimension entirely after his indefinite stay in Hell (He was possibly sent there by demons, like the Resurrector sent him to Earth in Doom 64). There he is captured by the world's inhabitants, the Argenta, and brought before the Order of the Deag in Sentinel Prime. The Argenta were unable to understand his language and he was incoherent, as his mind was crippled with rage and he only talked about demons. The Priests decided to put him through a trial by combat in the Blood Arena in order to judge on whether he would gain his freedom. Eventually, the Marine (dubbed as the "Outlander" by the Argenta) proved himself to be extraordinarily skilled and earning the admiration of the Argenta.

This garnered the attention of the clergy and ruling powers of Argent D'Nur, including the Khan Maykr herself, and despite being an outlander he was taken in and trained in their military, serving to spread the teachings of the Maykr race. Inevitably, Argent D'Nur was invaded by the demons and the Marine immediately entered the fight; in which his fervor for their destruction soon caught the attention of King Novik, who deemed him worthy to join the Night Sentinels. Initially the traditionalists of the Night Sentinels were very reluctant to allow the man into their ranks, as the Order forbids from having Arena-borne prisoners or foreigners. But over time, the Outlander earned their utmost respects as he proved his skills and unwavering determination at slaying Hell's demonic legions and becoming the Order's brother-in-arms.

The Marine participated in a pivotal battle against the Dreadnought, a monstrous Titan sent to lead a total demonic assault on the heart of Argent D'Nur. As the Sentinels were nearly overwhelmed by the demons, the Marine was approached by a Maykr, known as the Seraphim, who offered his help to defeat the Dreadnought. He was brought to the Chapel of Purity and submitted to the Divinity Machine, blessing him with fierce speed and great power to match his will. Using his newfound powers, the Marine took the Crucible, a weapon held for a true Sentinel warrior king, and using it to slay the Dreadnought and planting the blade inside the beast's heart. Since then the Sentinels deemed the Marine as a chosen hero, and he gained his (in)famous title: the Doom Slayer.

The Slayer lead the Night Sentinels deeper into Hell, earning many victories and unwittingly allowing the Khan Maykr to build factories in Hell's domains to harvest Hell Essence to produce Argent Energy while endorsing the Slayer's name as the will of the Maykrs. The Slayer did not care about the polity of the Sentinels and the Maykrs, as he was too focused on destroying the demons, and consequently remaining unaware of the Maykrs' insidious secrecy of harvesting Argent Energy from mortal souls at the cost of working with the demons. After the truth of the Maykrs' betrayal was revealed, the Slayer remained loyal to the Sentinels opposed to the Khan Maykr.

After years of fighting in a civil war against both demons and the loyalist who sided with the Maykrs, an opportunity to end the war was supposedly discovered. At the behest of the Order of the Deag, the Doom Slayer accompanied the Night Sentinels to Hell in the hopes of securing a final victory against the demons. However, they realized too late that the priests had betrayed them to the Khan Maykr. The Slayer and the Night Sentinels were left stranded in Hell and endlessly fought the infinite demon hordes. One by one, the Night Sentinels were killed, save for the Doom Slayer and Valen, the latter of whom allowed the demons access to the Elemental Wraiths in exchange for his son's resurrection. When Argent D'Nur fell, Valen, now known as the Betrayer, chose to stay in Hell for the remainder of his days in self-imposed exile. The Doom Slayer, on the other hand, continued his endless killing spree.

Over time, the demons themselves grew to fear him. A Titan was chosen as Hell's champion and sent to kill the Slayer, only to be killed by him. Having failed to kill the Slayer at every attempt, the demons instead lured him into a Blood Temple before collapsing the entire structure on top of him. Trapped beneath the ruin, the Slayer was entombed within a sarcophagus and frozen in stasis within the Kadingir Sanctum (It is unknown why the demons didn't kill him). He would remain there for a extremely long time until he was found and recovered by an Union Aerospace Corporation expedition led by Dr. Samuel Hayden while procuring demon specimens and Hell artifacts. Hayden led his forces in defeating the demons that fiercely guarded the sarcophagus and taking the Slayer to the UAC facility on Mars. Upon identifying the Slayer and his armor from within the Helix Stone, and the developing suspicion of his employee and former protégé Olivia Pierce, Samuel had the sarcophagus hidden in a concealed location within the UAC facility. Upon suspecting Olivia's pact with Hell and her plans for the Mars facility, Samuel intended to release the Doom Slayer in the event of the UAC losing control of its demonic experiments.

Doom (2016)
The Slayer is awakened after an unknown amount of time by Samuel Hayden during a demonic invasion of Mars triggered by Olivia Pierce. Upon breaking out of the facility he proceeds to Resource Operations to repair a satellite dish in order to get a complete picture of the situation. Hayden stops the Slayer from accessing the information in question and guides him to a VEGA terminal in order to give the Slayer an argent cell to improve his suit. The Slayer proceeds through the Foundry, stopping a meltdown by activating the terminals and then onto the Argent Tower itself. Despite instructions from Hayden, the Slayer smashes the filters channeling Argent energy to stop Pierce from opening a Hell portal. Despite this, upon reaching the top of the tower and confronting Pierce she uses an Argent Accumulator to break open the rift into Hell and warping the Slayer to Kadingir Sanctum.

He fought his way through the Sanctum to the location where his Sarcophagus was located in order to use the portal equipment left from a manned UAC expedition. Upon returning to Mars he finds the UAC facility overrun with demons as the demonic portal allows them easy access to the red planet. He finally meets Hayden face to face and Hayden instructs him in finding the Helix Stone to locate the Well and ending the Hell invasion, as well implanting a program into his suit that will allow him to teleport the Slayer back to Mars. The Slayer traveled to the Advanced Research Complex and retrieves the BFG 9000 before proceeding into the heart of the cult activity on Mars. He proceeds to the Lazarus Labs and fought the Cyberdemon before using the accumulator that powered the boss demon to teleport him back into Hell. He then fought his way through the Titan realm and the Necropolis on his way to obtain the Demonic Crucible. After defeating its guardians he retrieved the Crucible and is greeted by ghosts of the Sentinel Knights. Before any exchange can happen he is teleported back to Mars specifically to VEGA's primary facility. In order to reach The Well and cut off the power source keeping the portal on Mars open he has to overload VEGA's systems to rip open a portal to the Well. Before overloading VEGA's core however, the Slayer pauses and makes a copy of VEGA onto a backup device he takes with him.

Finally, at the Well the Slayer fought his way to each of the Wraith bodies and uses the Crucible to absorb their energy to cut off the flow to the portal. Once the final Wraith has been disabled the Slayer is greeted by several Sentinels who stand guard around a pit leading to the final battle. Here he meets Pierce, bloodied and in shock that she was betrayed by her demonic masters. These same masters then hit her with a crimson lightning bolt, turning her into the Spider Mastermind, though it is ultimately killed by the Slayer. After this Hayden pulls the Slayer back to Mars and keeps him restrained by the teleporter. He takes the Crucible from the Slayer, stating he would use it to continue the production of Argent energy for humanity's benefit. Knowing the Slayer would attempt to stop this plan, Hayden teleports the Slayer away to an unknown location.

Doom Eternal
''As a new demonic threat erupts on Earth, so does the Slayer, his iconic Praetor Suit modified and upgraded for increased mobility and destructive capability. For the Slayer, it is a battle that has continued for an eternity, a fight that can only be won by confronting his past if he hopes to stop history from repeating itself.''

''VEGA has created an inter-dimensional man-machine gateway that allows the Slayer to engage in skirmishes across the multiverse directly. As the Slayer enters the gateway, his surroundings take the form of an arena, in tandem with the arrival of adversaries ready for battle Seemingly extracted from the data of his experiences, the opponents the Slayer faces are familiar but now transformed by some unseen force.''

''With further analysis Vega observed ephemeral distortions within the fabric of reality; scrambled reflections of the physical world. As the slayer interacts with this anomaly, a peculiar ripple in the anomaly's composition emerges, seemingly reacting to the Slayer's presence reflexively. The anomaly reshapes itself, calibrating itself to the Slayer, responding to his memory, ancestral, and memetic data, before ultimately projecting itself onto the Slayer himself.''

Following the events on Mars, the Doom Slayer returned from his banishment with the Fortress of Doom controlled by VEGA and finding Earth invaded by demons. Seeking to end the demonic invasion, the Slayer sought to eliminate the masterminds of the invasion, the Hell Priests Deags Nilox, Ranak, and Grav, while also coming into conflict with the Khan Maykr, who authorized the consumption of Earth in order to harvest the souls of mankind into Argent Energy, following the destruction of the Well.

The Slayer first killed Deag Nilox before confronting Ranak and Grav, displaying their dead colleague's decapitated head before them. The Khan Maykr intervened, teleporting Ranak and Grav away and failing to convince the Slayer to give up in his mission. The Slayer traveled to the Sentinel world of Exultia to retrieve the Celestial Locator that will allow him to find the remaining Hell Priests. There, he encounters the ghost of King Novik, who also tells the Slayer to to give up his crusade against Hell and the Maykrs, claiming Earth is no longer his to save. The Slayer then travels to Hell to retrieve a power source for the Celestial Locator from the Betrayer, who also request the Slayer to kill his son that has become the Icon of Sin and gives him a dagger to carry out the deed.

With the Celestial Locator, the Slayer tracked down Ranak in the former UAC Arctic facility, defeating his Doom Hunters, and decapitating the hapless Hell Priest with the Doomblade. His actions caused the Khan Maykr to move Deag Grav to Sentinel Prime. Following VEGA's suggestion, the Slayer embarked on finding Samuel Hayden to find the last Hell Priest. Traveling to the ARC Complex, the Slayer roughly retrieved Hayden's damaged robotic body and recovering the Crucible; in which Hayden had anticipated his return and instructed his ARC caretakers to give the blade to the Slayer. While retrieving Hayden, a corrupted night sentinel, known as a Marauder, approaches and attacks the Slayer only to meet his end shortly after. Upon uploading Hayden's mind into the Fortress of Doom, the Slayer learns from him of Grav's location and that the only access to Sentinel Prime is through a portal in the the city of Hebeth in Mars' core. The Slayer travels to a UAC facility on Phobos where he uses the massive BFG-10000 to blast a hole in Mars, which he uses to reach Hebeth. After arriving in Sentinel Prime, the Slayer fought and killed the Gladiator, and killing Grav by blowing his head up with his Super Shotgun. However, Deag Grav's death on sacred ground cost the Slayer to be excommunicated from the Night Sentinels.

Following the death of the last Hell Priest, the Slayer was temporarily trapped on the Fortress by the Khan Maykr, who remotely shut down the structure to prevent the Slayer from continuing his interference and revealing her intentions to awaken the Icon of Sin. However, the Slayer returned power to the Fortress by using the demonic Crucible as its power source, and later traveling to Argent D'Nur to retrieve his Crucible that he used to slay the Dreadnought and using it to stop the Icon. As the Crucible's blade keeps the Dreadnought from returning to life, the Slayer only took the Crucible's hilt and using it to forge a new blade.

Guided by Hayden, the Slayer fought his way to the roof of Nekravol and using its portal to travel to the Maykr realm of Urdak, where the Khan Maykr prepares to waken the Icon. He interrupts the awakening ceremony and plunging the Betrayer's dagger into the Icon's heart; this destroys the heart and freeing the Icon of Sin from the Maykrs' control, and causing the massive demon to destroy the dimensional barrier between Urdak and Hell, thus allowing the demons to break their pact with the Maykrs and directly invading Urdak. The Slayer fought and killed an enraged Khan Maykr before returning to Earth to confront the Icon, but is forced to leave VEGA on Urdak in order to open a portal to Earth. Following an intense battle, the Slayer kills the Icon by stabbing the Crucible in its head. With the Icon dead, along with the Hell Priests and the Khan Maykr, demons were momentarily defeated.

The Ancient Gods - Part One
Despite having defeated the Icon of Sin and halting Hell’s invasion of Earth, the Doom Slayer’s victory over the demons did not come without cost. The death of the Khan Maykr and Hell’s conquest of Urdak have given the demons a chance to dominate all dimensions and reinitiate their invasion of Earth. To prevent this, the Doom Slayer, along with Samuel Hayden and ARC scientists, embark on a mission to find and liberate the Slayer’s ally, the Seraphim. Traveling by sea on an ARC Carrier towards the UAC Atlantica Facility, the Slayer fights his way to the Seraphim's containment pod. When Hayden requests he be uploaded into the pod, it is revealed that he and the Seraphim are one and the same.

After returning to the ARC Carrier, the Seraphim tasks the Slayer to find and retrieve the Father’s Life Sphere from the Blood Swamps of Hell in order to return the Father to physical form. After fighting his way through the Blood Swamps, the Slayer finds and retrieves the Father’s Life Sphere. However, the Slayer chooses to destroy the sphere rather than hand it to the Seraphim and instead retrieves the Dark Lord’s Life Sphere before returning to the ARC Carrier.

Despite most of the Carrier’s crew evacuating upon seeing the Dark Lord’s Life Sphere, a lone Intern stays to help the Slayer reach Urdak assuming that the Slayer intends to resurrect and destroy the Dark Lord, which in effect will destroy all demons. Upon reaching the corrupted Urdak, the Doom Slayer fights his way to the Luminarium where anyone who has a life sphere may activate it. However, the Slayer is confronted by the Seraphim upon reaching the Luminarium’s entrance. Consumed by Urdak’s demonic corruption, the Seraphim is transformed into a demon and after a lengthy battle is ultimately defeated by the Slayer and is teleported away by the Father. Despite being warned that bringing the Dark Lord into physical form is irreversible, the Doom Slayer proceeds to summon him.

The Ancient Gods - Part Two
The DLC picks up directly after the ending of Part One, the Slayer fires a shot at the Dark Lord, but the Dark Lord informs him that no blood can be spilt in the Luminarium, and that, if the Doom Slayer survives, he will be challenged in ritual combat in the capital city of Hell, Immora. The Dark Lord leaves and the Slayer is transported to Argent D'Nur, where he fights off the hordes that attack him. He lights the torch of kings, an act that is strictly forbidden by the Maykrs, telling the inhabitants of Argent D'Nur and the remaining tribes of Sentinels that he seeks a Wraith crystal to power the Gate of Divum. He travels on his Dragon to the World Spear, where he meets the hologram of Valen, who gives the Slayer his (now blue) Sentinel Hammer. The Slayer discovers that the World Spear is not a crystal, but a colony ship, housing thousands of Wraiths.

He takes the Wraith crystal, and heads back to a reclaimed Earth, whose invasion has stopped due to the death of the Icon of Sin. Fighting his way through the demons, he arrives at the Gate of Divum, activates it, and travels to Immora, where the Dark Lord is located. The Slayer makes his way past the demons outside the gate, and through the non-demonic people inside, goes through one final portal, where he meets the Dark Lord.

He and the Slayer fight, revealing that the Father was not the creator, and the Dark Lord, in fact, made Jekkad, then the Father, then the Makyrs and all other realms to find the cure for mortality. VEGA, the Father, confirms all of this to be true. Davoth is defeated, and asks the Slayer if he has anything to say before killing his creator. the Slayer responding "No." After the Dark Lord dies, all the demons outside of Hell disappear, the Doom Slayer falls unconscious due to his connection to Davoth through the Divinity Machine (which was powered with a piece of Davoth). He is put into a sarcophagus by Maykrs until he is needed again.

UAC REPORT FILE H8UM66S
In the first age, in the first battle, when the shadows first lengthened, one stood. Burned by the embers of Armageddon, his soul blistered by the fires of Hell and tainted beyond ascension, he chose the path of perpetual torment. In his ravenous hatred, he found no peace, and with boiling blood he scoured the Umbral Plains seeking vengeance against the dark lords who had wronged him. He wore the crown of the Night Sentinels, and those that tasted the bite of his sword named him... the Doom Slayer.

UAC REPORT FILE 9REIZDUR
Tempered by the fires of Hell, his iron will remain steadfast through the passage that preys upon the weak. For he alone was the Hell Walker, the Unchained Predator, who sought retribution in all quarters, dark and light, fire and ice, in the beginning, and the end, and he hunted the slaves of Doom with barbarous cruelty; for he passed through the divide as none but the demon had before.

UAC REPORT FILE HR93TE1F
And in his conquest against the blackened souls of the doomed, his prowess was shown. In his crusade, the seraphim (angel) bestowed upon him terrible power and speed, and with his might, he crushed the obsidian pillars of the Blood Temples. He set forth without pity upon the beasts of the nine circles. Unbreakable, incorruptible, unyielding, the Doom Slayer sought to end the dominion of the dark realm.

UAC REPORT FILE I3S5A9XB
The age of his reckoning was uncounted. The scribes carved his name deep in the tablets of Hell across eons, and each battle etched terror in the hearts of the demons. They knew he would come, as he always had, as he always will, to feast on the blood of the wicked. For he alone could draw strength from his fallen foes, and ever his power grew, swift and unrelenting.

UAC REPORT FILE ZPHVM41A
None could stand before the horde but the Doom Slayer. Despair spread before him like a plague, striking fear into the shadow-dwellers, driving them to deeper and darker pits. But from the depths of the abyss rose The Great One, a champion mightier than all who had come before. The Titan, of immeasurable power and ferocity. He strode upon the plain and faced the Doom Slayer, and a mighty battle was fought on the desolate plains. The Titan fought with the fury of the countless that had fallen at the Doom Slayer's hand, but there fell the Titan, and in his defeat, the shadow horde was routed.

UAC REPORT FILE WMHVVFJ
And in his terrible rancor between worlds and through time, the Hell Walker found the wretch who shall not be named, but in his heresy was loyal to his evil cause. The wretch adorned the Doom Slayer in a mighty armor, wrought in the forges of Hell, impenetrable and unyielding. With sword and shield of adamantine strength, the Doom Slayer set to banishing all that was left unbroken by his savagery to the void.

UAC REPORT FILE JSLR7ESL
Yet as the mighty Titan fell and dread engulfed the armies of Doom, the demon priests of the Blood Temples laid a trap to capture this scourge of Hell. Insatiable, even by the vanquishing of the Great One, the Hell Walker sought prey in the tombs of the Blood Keep. And blinded by his fervor, the lure drew him in. The priests brought down the temple upon the Doom Slayer, and in his defeat entombed him in the cursed sarcophagus. The mark of the Doom Slayer was burned upon his crypt, a warning to all of Hell that the terror within must never be freed. There he lies still, and evermore, in silent suffering.

Personality and traits
Unlike some of his earlier appearances in the series, he shows a little more emotion, expressed more through actions rather than words. During his crusade and his interactions with Samuel Hayden (who tries to use and ask the Marine for help) when the latter tries to justifies the UAC's actions in Hell as being in mankind's best interests despite the evident risks and aftermath consequences, the Marine responds by angrily dismantling communication equipment at best, and flat-out destroying key UAC equipment at worst despite Hayden's insistence of not doing so. This makes it clear that the Doom Slayer's motivations and priorities start and end with stopping and killing demons and not for the UAC and their economic interests, and very little for anything else. He does, however, show to be sympathetic to a degree as seen when he made a backup copy of VEGA before destroying it, although this could be attributed to the fact the AI opted to sacrifice itself to aid the Doom Slayer in his quest. As such it can be deduced the Marine can show concern for those that either aid him on his demon-slaying mission or otherwise show the necessary resolve to act for the real greater good. Also, despite killing the Khan Maykr, Slayer understands that she doing what she did was for the sake of her race, yet his resolve remains untouched. It's clear that the Slayer cares for humanity in some way, as he is willing to sacrifice not only his relations with the Argenta but also destroy the realm of Urdak in order to stop the demonic invasion of Earth. Despite his war with the demons he almost never attacks their human allies simply expecting them to get out of his way which they often do. This attitude does not extend to other species or even his allies as he is quick to neutralize them if they would get in his way as seen with his actions directed at The Father and the Seraphim.

The Doom Slayer also appears to be quite intelligent as his original Praetor Suit could be found disassembled in his quarters with numerous parts of it repurposed for his new Praetor Suit, which would make sense given he is the same marine from Doom 64, who had the role of an engineer.

Unlike the first two games, the Doom Slayer never makes any vocal utterances in Doom (2016), even when taking damage or dying. This seems to change to slight grunts in Doom Eternal. However, in Doom Eternal, and for the very first time in the series of games, the Doom Slayer's speaking voice is heard during flashbacks upon entering Sentinel Prime, though at this point having nearly lost his sanity and ranting about killing demons: "...guts, huge guts! Kill them... must kill them all!" According to the Story of the Sentinels codex entries, the Slayer used to speak but later chose not to for reasons unknown aside from uttering "guttural hefts of anger" on his enemies.

Quake Champions Character Background
The UAC's most remarkable discovery was in Hell's Kadingir Sanctum. Surrounded by nearly impassable terrain, the team discovered a sealed tomb – its entrance and walls covered in protective runes and imprisonment incantations.

The tomb revealed many artifacts, including the Praetor suit. Most notable, however, was an enormous sarcophagus bound to the center bedrock of the tomb with thick iron bands – seemingly anchored to prevent anything from getting in or out.

It was believed this sarcophagus must hold a rare or powerful demon, but when later opened, it revealed the body of a man. Unrelenting; unforgiving; driven by a rage that cannot be contained. The Doom Slayer has been to Hell and brought it to its knees. He has no records, and he seems to vanish as quickly as he appears. The few witnesses account available detail a ruthless killer. Long ago across worlds and time, in another universe, Doom Slayer (B.J. Blazkowicz (Doom)) had become a great hero when he saved earth from an invasion by demons from hell. He continued to carry the left hind foot of his pet rabbit Daisy, killed by the marauding demons when their invasion of Earth began. He carried it as a reminder of innocence lost, not for luck--after all, it wasn't lucky for Daisy. The armor he wore became iconic, and it changed the world.

At some point the wide-eyed Doom Marine, found himself again newly transferred to Mars, the soldier got more than he asked for when Dr. Betruger betrayed the UAC.

During his battles to protect Earth, he found himself trapped in hell fighting an eternal battle with the demons. He would travel across time and worlds to save the multiverse. Sometimes finding himself pulled to battle other Champions from other worlds and alternate earth in the Arena Eternal (in this earlier period, he received new armor, that fit like a glove). He continued to be a great champion defending many worlds from the demons and killing them in Hell.

The Slayer's Testaments described him as an ancient and mystical time-traveling warrior called the "Doom Slayer" or "Hell Walker" who was either banished to Hell or chose to stay there. The Doom Marine proceeded to rampage his way across Hell for "eons" as an unstoppable force of death and destruction, becoming an icon of fear to all demons.

His one-man war was only ended after he was lured into a trap, allowing the forces of Hell to collapse one of their temples on top of him. Entombed inside a stone sarcophagus, he was sealed away in the Kadingir Sanctum, hopefully forever. Unfortunately for the demons, the Doom Marine's sarcophagus was eventually retrieved by the UAC (on an alternate earth) and brought back to Mars. Sure enough, after Hell commences its invasion of Mars, the Doom Marine awakens as bloody-minded as ever. He then arms himself with a pistol, reclaims his Praetor Suit and begins to fight the hordes of hell once more. After he saves Earth, he is captured by Hayden and put into stasis again, while the scientist prepared his next plans.

With regards to Quake canon, Champions games is a prequel to the Arena games (Quake III Arena/Live). Though with regards to when the Doom Marine was pulled into the Arena Eternal are roughly switched chronologically. That is that Doom Slayer entered into the Arena Eternal during the earlier Champions competition, whereas an earlier incarnation code-named "Doom" entered in the later Arena competitions.

DOOM (1993)/Thy Flesh Consumed
B.J. and his squad are sent to Phobos base after the UAC's teleportation experiments go wrong and Hell breaks loose on Mars' moons. A team including Doomguy go to check it out, but due to a demonic invasion, everyone but Doomguy dies, so it's up to him to take care of business. Doomguy takes the fight through a portal to Hell where he defeats the Spider Mastermind and returns to Earth. Upon returning to Earth, Doomguy finds it already ravaged by demons, who decapitated his pet bunny Daisy and murdered his wife and child. He fights the forces of Hell the Spider Mastermind had sent through ahead and takes revenge for his family and dead bunny.

Doomguy continued to keep a torn photo of his family and a lucky rabbit's foot taken from Daisy upon him.

He has since gotten a new, much living Daisy.

DOOM II/No Rest for the Living/Final Doom
After the battle on Mars' moons, Doom Marine takes leave and returns to Earth, only to discover it has been invaded.

The survivors plan to flee the planet, but the only remaining functional spaceport is under Hell's control. He fights to take back the spaceport and the remaining humans succeed in fleeing to space. Doomguy then travels to the demons' entry portal on Earth to close it and succeeds after killing the Icon of Sin. Humanity then returns to Earth to rebuild.

Final Doom covers the Doom Marine's (or possibly another character's) destruction of two other Icons of Sin, one after Hell's invasion of Jupiter's moon Io (TNT: Evilution) and the other after another invasion of Earth (The Plutonia Experiment), both invasions being brought about by further experimentation with interdimensional gateways.

How the Doom Slayer travels through Worlds and Time might be the least important aspect of his history. Just that roughly all the games appear to take place in a single 'earth' timeline in some fashion or other. Parallel earth have not yet been confirmed in Doom series itself, though possibly hinted at in the Art of Doom Eternal (although its part of Quake Champions', and MachineGames universe's stories, see Young Blood (B.J. mentions having traveled to or seen several Earth dimensions)).

DOOM 64/Lost Levels
Something survived on Mars' moon, however. The Mother Demon works to resurrect the fallen armies of Hell. Doomguy returns to Mars' moon to take out the trash, journeys into Hell again and kills the Mother Demon. This time, however, he closes the portal from the inside. He has decided to remain in Hell to keep the demons from ever escaping it again.

It's sister, the Resurrector takes revenge on the Marine, by sending him away back to Earth, and attempts to kill him. He fights his way through another UAC base, works his way back into Hell, and takes revenge on the sister. He decides once again to find a perpetual and eternal path through DOOM, slaying more demons of Hell.... (note the quote from the Lost Levels ending is based upon a similar quote in the Slayer's Testament, and intentional link between the two games' stories)

Pre-DOOM 2016
In the time of King Novik, as the Argenta secured peace and safety through dimensions across time and space, Doomguy is found by the Night Sentinels in a valley outside the castle walls on Sentinel Prime. How he arrived is unknown, though he was likely sent by demons to get rid of him. Sentinel scouts had found the Outlander bloodied, clawed and near death, mumbling of impending Doom and the forces of Darkness. The scent of blood followed him, and the gore that stained his armor seemed not entirely of his own. By Argenta law, he would be judged in the Coliseum, where he would be given the chance of all who stray from the Path to fight for his freedom.

His mind seemed crippled with rage. His will overcame his injuries, and in the Blood Arena, he proved his worth. His technique was crude and brutal, but the determination in his charge echoed that of any trueborn Sentinel. His war cry echoed through the Coliseum as did the sound of his fury, and the guards cheered his banner, 'Rip and Tear' they shouted as the beast pushed beyond mortal wounds and certain death.

He would be gifted no rank, no title. The survivors in the Arena were provided only one reward - the right to earn an honorable death while spreading the blessings of the Maykrs to those in need.

Doomguy's determination was witnessed, his ravings documented, and through the word of the Order, the Khan Maykr had him brought before her. She set her minions to learning his tongue, for with his ugly words he spoke of lands unseen, creatures born of fire and a dark place unknown to the Queen and her cast. Yet another opportunity to expand the gift of the Maykrs to those in need.

On the eve of the Black Star, the Dark Ones came from a world beneath our own, not through ship nor ephemeral vessel but through the fabric of dimensions. Out of swirling, fiery gates came horned beasts from a timeless realm. First one, then many, crashing waves of evil swelled from the obsidian forest of the Argentian Overlands. The Alorum clans from the eastern mountains fell first to the black hordes of devils, and the city of Telorum was slung under the weight of their charge.

He and the Sentinels rose to meet the beasts with a spear in hand, the might of the holy fleet striking back at the demons, but the dark gates from whence they came gave birth still to even greater hordes.

A line of blood was drawn with the fallen on both sides, and the Unholy Wars began as the time of darkness came upon us, thus beginning the first age. In the 1st Age, Hell constantly strove to take over Argent D'Nur in order to harness sentinel energy and expand their dominion.

The Crowned leader of the Night Sentinels loses his son on the battlefield. In return for his offspring's revival, he betrays the Sentinels and the city by guiding the traitorous Deag Grav to the Wraiths. The Hell priests put a curse on their patron deities to siphon their homeworld's energy. To which (in a secret deal forged between the Maykrs and Doom overlords) would blend Sentinel Energy with demonic essence in conjugation with the tormented souls of the fallen Argenta to produce the more powerful Argent Energy.

Valen's son was returned to him, but in a twist to their bargain had instead been resurrected as the Icon of Sin.

The demonized former sentinel tears the Argent City apart while everyone watched helplessly. To make matters worse, the Slayer and his contingent of Sentinel warriors ventured deep into the heart of Hell itself in order to bring the fight to their enemies' heartland.

Only for the noble warrior creed to find out the horrible truth behind the secret of Argent Energy. Not only was the fuel source utilized by the Argenten people fueled by the suffering and defilement of their fallen brethren; whose corpses were later transformed into legions of the damned after their souls had been processed.

The Order soon realized they had been betrayed from within by the Argenten people's very patron divinity; the Khan Maykr. It is revealed she had in fact helped construct the malignant devices which the Legions of the Doomed used to harvest the souls of their enemies, the slayer's people, her subjects.

In order to process and refine their tormented souls and concentrate their inherent quintessence into refined Argent Energy. After they had returned to their homeworld to convey such a horrendous finding, the Night Sentinels found themselves rejected by their former capital. As the Deag; now going by the moniker of Hell Priests, corrupted by their exposure to demonic essence and the influence of the Kahn Maykr. Had turned Sentinel society against those who knew the awful truth.

Wronged, betrayed and consumed with rage, seeking vengeance, retribution, and redemption, the Slayer decides to reenact his murderous crusade on Hell alongside the resistance sect known as The Loyalists. Samur Maykr, also known as the Seraphim, would spirit him away from prying eyes in order to give him immense power (in Christian mythology, a Seraph is the highest rank an angel can achieve).

In the time that followed, however, another great Titan had been dispatched by the armies of Doom to waylay another Argenta city heralded by the Night Sentinels.

In an epic battle that waged for an untold period; the Doom Slayer would rise to the occasion wielding his own Crucible Sword and strike the towering behemoth down. In awe of the foreigner's might and menace; the people of Argent D'Nur crowned him as their latest king, inaugurating their savior with the moniker of the Doom Slayer. While trapped in Hell by yet another betrayal from the Hell Priests, an entity known only as "The Wretch" would seek the Doom Slayer out. He would enhance his armor into the Praetor Suit so that the Slayer can better survive in Hell with his newly obtained inhuman powers. As time may flow differently in this dimension, the Doom Slayer torments Hell for thousands of years long after the remnants of his Loyalist sect died out.

The demons, unable to defeat him, instead devise a way to contain him. After being lured into a trap at Kadingir Sanctum, he is captured by the forces of Hell, separated from his Praetor Suit, and trapped in a sarcophagus, sealed by demonic power.

DOOM (2016)
Samuel Hayden retrieved the Doom Slayer from Hell in case the demonic forces escaped onto Mars, which they did. He awakened him to stop a demonic invasion of Mars instigated by his former protégé, Dr. Olivia Pierce. He is released from the sarcophagus, supplied with his Praetor Suit, and sent to work. He journeys between worlds, destroying the Gore Nests that serve as Hell's links to our world. Samuel Hayden guides him through slaying the demons on Mars and Hell, equipping him with a dimensional tether allowing him to travel between them. The Doom Slayer, disapproving of Hayden's deadly Hell energy-siphoning operations, destroyed the Argent Energy filters he asked him to carefully remove. The Doom Slayer eventually retrieved the Crucible, used it to free the Wraiths from their subjugation and absorb their Argent Energy, and killed Olivia Pierce. Then Hayden uses the tether to return the Slayer to him, incapacitate him, and take the Crucible from him. Hayden explains that even though his attempts to siphon energy from Hell resulted in a massive loss of life, he believes the massive energy crisis on Earth it's solving makes the ends justify the means. He claims that even though the Slayer destroyed the filters, he can rebuild his operation with the Crucible, which he reveals to be a long blade of pure Argent Energy. To prevent the Doom Slayer from trying to stop him, Samuel uses the tether to send him to a place where he can do no harm and where no harm will come to him.

Quake III Arena/Quake Champions
The Doom Slayer is sent to the Arena Eternal, a series of battlegrounds where the Vadrigar have taken the greatest warriors from across time and space, and even parallel universes, to kill each other in endless gladiatorial combat for their amusement. Every time the gladiators are fragged in combat, the Vadrigar revive them so they may fight again. In Quake canon, Champions is a prequel to Arena. The background of the Slayer in Quake Champions includes lore taken from Doom (2016), Doom Classic/Doom II/Doom 64, Doom 3, and the original Quake Arena games.

DOOM Eternal
The demons have invaded and devastated the Earth, just as they did in Doom II. After traveling to Earth from the Fortress of Doom in orbit, the Doom Slayer must use his upgraded Praetor Suit and Super Shotgun and a wide range of weapons both old and new to stop them.

It is confirmed that Doom Slayer is one of the generals of the Night Sentinels and human whereas the rest of the Night Sentinels are of the Argenta race. In the distant past, after leading a successful battle against the demons of the Blinded Order, his army was killed by a group of demonic dragons. As he was about to be killed after a dragon tossed him into the air and he fell thousands of feet below, he was rescued by the Seraphim.

During the betrayal of Valen, the Doom Slayer led the remnants of the Night Sentinels that were still loyal to his cause to destroy the foundry of souls, the source of the Khan Maykr's power to turn the tide of the war against the forces of Hell and the corrupt Argenta. Deag Grav eventually trapped and stranded the Slayer and his warriors in the void.

It's revealed in flashbacks that after being found by the Night Sentinel patrols, he was brought to their arena on Sentinel Prime to be judged whether he is worthy to be a warrior of the realm or not by Ranak and Grav. He eventually succeeded in proving his worth and under the order of the Khan Maykr, he was made into a Night Sentinel, eventually becoming one of their warrior-kings.

Due to his exploits in exterminating demon hordes on Earth, most people regard him as humanity's avenging angel and savior while the UAC and Allied Nations try their best to deny his existence. The resistance refers to him as the Doom Slayer and sometimes the Doomguy.

Powers and Abilities

 * Transcendent Human Physiology: The Khan Maykr states that the Doom Slayer was once a mortal. Despite fighting with demons for centuries, the Doom Marine did not grow old or lose any of his strengths; he also no longer needs food, drink, or even rest, becoming something akin to an immortal.
 * Immense Physical Condition: Despite being human, the Doom Marine has shown to possess immense physical prowess which is far beyond human and even most demons. After the blessing of The Seraphim he gained even greater power matched only by his own will, greatly empowering him to even more impressive levels. These abilities are all further augmented by the Doom Marine's Praetor Suit and are used when fighting against the forces of Hell.
 * Immense Strength: The Doom Slayer´sstrength allows him to lift heavy objects and throw them on great distance without effort, is able to smash through walls, and carry a great amount of various heavy weapons without tiring. His might is sufficient to grapple and overpower superhumanly strong demons in hand-to-hand combat, and perform visceral "Glory Kills" to ensure their complete destruction.
 * Immense Durability: Even before his blessing, Doomguy was extremely durable, even on human terms, being able to survive injuries which would kill an average person and even weaker demons. After his transformation and gaining his praetor suit, he becomes near-unstoppable, being able to endure both cold and heat of the Hell, is capable of enduring the hits that the Demons are able to land even of the strongest one as well as falls from great heights which could kill wearer-class demons unfazed.
 * Immense Speed, Agility, and Reflexes: Even with his heavy armor and equipment, the Unchained Predator is highly mobile with amazing reflexes and reactions. He can outmaneuver his opponents with great dexterity, avoiding their attacks, he is able to jump to great distance both horizontally and vertically and land from heights perfectly. He can also very quickly switch between his weapons.
 * Immense Stamina and Endurance: The Slayer's most significant feature is his incomprehensible stamina and endurance which are further strengthened by his will. When he was found injured in Sentinel Prime and was sent to the arena, regardless of his injuries he was not only still able to fight, but he managed to win all his battles, gaining admiration from future Hell Priests and even the Khan Maykr herself. After his blessing, he can operate continuously without any rest, food or water.
 * Argent Energy Absorption: The Doom Slayer can absorb and metabolize Argent Energy via his Praetor Suit, which reroutes it throughout said combat fatigues subsystems and into his body. This allows him to increase either his Health, Armor or Ammo to maximum levels.
 * Destruction Empowerment: The most unique thing about the Doom Marine is his ability to grow stronger and become even more tenacious with every demon he kills. According to the game's lore, this is the result of Samur Maykr giving the Slayer the ability to absorb the souls of those he killed.
 * Powerup Usage: Whereas powerups (such as Berserk and Haste) in the game are described as killing or driving human users mad, the Hell Walker can use these without any trouble, sustaining full control and not taking any damage to either his body or mind from using such artifacts multiple times.
 * Rune Absorption and Usage: Through some unknown means, the Slayer is able to use mystic runes that he gains upon completing trials. Once a trial is complete, the Marine can then use a rune to gain new abilities.
 * Advanced Intellect: Due to his time spent with technologically advanced civilizations, both within his own home dimension as a Space Marine and in the coming years with the Argentan people before their descent into corruption, the Slayer is able to use technology from other worlds with no signs of unfamiliarity, despite resorting to violence. He has also an adept study of computer sciences as the lore of Doom Eternal, as well as literary paraphernalia within the Fortress suggests. After uploading VEGA's core memory into the Maykr/Argenten stronghold, the Marine had been maintaining its system functionality; showing a keen tech-savviness behind his otherwise brutish approach. He also improves his Praetor suit with the help of VEGA as well as creating his own ballista and training room for keeping his skills sharp.
 * Expert Combatant: The Unchained Predator is a highly trained and experienced combatant, even when lacking ammo and weapons, he can slay demons just with his bare hands alone, without the need of any melee weapon, he can defeat and tear apart demons far bigger and stronger than him just with his own skills in hand-to-hand combat. Due to his training with the Night Sentinels he became quite disciplined and capable of focusing his rage and become proficient with swords and spears. With the Praetor suit, these skills are further increased, making him a monster in both armed and unarmed combat. In his stronghold, he has his own special room where he can summons demons to still maintain and improve his combat abilities. He could overpower entire armies of hell on his own near-effortlessly whatever Earth's defense fails even in their best technology.
 * Weapon Mastery: The Doom Marine is capable of using any weapon or firearm in his possession with master proficiency. He can use all artillery he comes across with great effect, while also upgrading and modifying his arsenal. He is also capable of carrying a vast amount of weapons, carrying up to 12 weapons including special weapons (Chainsaw and BFG) and grenades. Also able to use swords and other bladed weaponry equally effectively due to his training and crusades with the Night Sentinels.
 * Indomitable Rage: The defining proponent of the Slayer's persona is his undying fury aimed against the forces of Doom. His rage is such that it was enough to keep him alive and propel him forward across worlds & time for the better part of an eternity after he sealed himself in Hell to fight off its accursed denizens. The intense hatred he feels towards the demons and all their affiliates have not only given him insurmountable strength to wade through waves upon waves of monstrous fiends and apparitions, but made him legendary in the annals of history across multiple dimensions as well as in their recorded fables.
 * Intimidation: Due to his appearance, actions and legend, the Slayer is greatly feared among mankind even though he never injures or kills any of them. The ARC soldiers were terrified when he came before them. Despite that most demons during battle are not frightened by his presence and attack him constantly; the lore entries show that all of Hell itself is terrified by the Doom Marine to an extent that they fought vigorously just to keep him sealed in his tomb when a UAC excursion team came looking for him. The Hell Priests and even the Khan Maykr were greatly terrified of him as well.
 * Indomitable Will: The Doom Marine possesses tremendous amount of willpower and determination that cannot be weakened. No matter the situation, opponents or obstacles he pushes forward, refusing to give up regardless of what happened. His determination to wipe out demons and protect humanity has remained with him constantly for years. King Novik describes the Slayer´s will as a sword which cut through his enemies.

Though it is only told by demonic recordings, the Doom Slayer had managed to kill Hell's greatest Champion 'the Titan', indicating how he became the most feared being to all of Hell's Legions.

Praetor Suit
This armor has carried the Doom Slayer through untold horrors. Not even the most powerful demons or fires of Hell can stop his advance. his armor also has a self-supporting trait which allows Slayer to survive underwater or in the vacuum of space.

Attachments

 * Deep Space Rebreather: Automatically regulates the air mixture for high and low-pressure environments.
 * High Impact Chest: Snaps into place on the chest to protect the wearer from high impacts.
 * Performance: Created for the Praetor Suit to maneuver effortlessly while in the air or in a vacuum.
 * Stabilizers: Helps the wearer to adjust movement while in the air.
 * Delta Jump-Boots: This early version of the Delta V Jump-Boots was not nearly as compact as the finished model, but the extra mass was still used to adjust movement in the air.

Arena
Though bare-bones, this early armor fits like a glove.

Attachment

 * Helmet Multi-Tool: Multi-tool with laser sighting and headlamp.
 * Shell Holder: Lots of demons require lots of shells. Shotgun ammo should be always on hand.
 * Arm Bandages: Protect the shoulders from damage and demon blood.
 * Thigh plates: Trade carrying space for increased protection.
 * Combat Knife: Used for last-ditch efforts.

Doom Marine
Recently transferred to Mars, this soldier got more than he asked for when Dr. Betruger betrayed the UAC.

Attachments

 * No Helmet: The standard-issue space marine helmet is airtight and durable, while still allowing a wide view area.
 * Flashlight: Pierce the darkness using this high-powered flashlight. Unfortunately, it is quite large and is almost impossible to use while firing.
 * Spare Mags: Keep ammo close at hand. You're going to need it.
 * Air Tanks: Doesn't offer additional protection but allows demons to see their death in your eyes.
 * PDA: Given to you on your first day on Mars. Allows you to read other people's email and hear their personal recordings.

Doom Guy
The original and iconic armor. It forever changed the world.

Attachments
Praetor Suit 2.0
 * Classic Face: Relive the old days using the face you know so well.
 * Down But Not Out: Bruised, beaten, and bloody. This is where the fun starts.
 * Advanced Helmet: A more sophisticated version of the base helmet.
 * Bandolier: Ready for war.
 * Arm Guards: For when fighting demons gets nasty.
 * Abdominal Plating: For when you need to hide your abs.
 * Leg Armor: Thick plating for the upper legs.
 * UAC Riot Greaves: Protects the shins from heavy impacts.
 * Spare Shells: Keeps shells handy in case of an emergency.

A new prototype based on previous armor augmented via VEGA and Slayer himself is now better and stronger than ever with more exclusive equipment and other suitable things.
 * Doomblade: Slayer now has a blade on his left arm which ejects out during Glory kill
 * Equipment Launcher: Attachable to the Slayer's left shoulder, the Equipment launcher is capable of shooting out Frag Grenades, Ice Grenades, and the Flame Belch
 * Delta Jump-Boots 2.0: Slayer carry boots which allows him to perform double jump while leaving him to adjust movement in the air.
 * Dash trusts: Thanks to thrusters, the Slayer now can move very swift on both to ground and air.

El Diablo Shotgun
This alternative to the Super Shotgun is engraved with the images of its victims. Well, after their skin was boiled off.

Doom 2 Shotgun
The DOOM 2 break-action double-barrel Super Shotgun has been to Hell and back. It's old, beaten, and packs a mean punch.

Faces of the Slayer
The face of the Doom Slayer has evolved and changed over the years, with the character originally having reddish/brown hair, but later having dark brown hair as of Doom: Eternal. The Doom Marine's face is obscured (in Doom (2016), Doom Eternal, and other games.

In Doom (2016), Doom Slayer's face, which was discovered through model and game texture rips, shows similarities to the Classic Doomguy's face. In classic maps he uses a texture based on his face from Doom. His facial model and jawline and general structure of his face resembles B.J. Blazkowicz from Wolfenstein: The New Order to some extent. This may be a reference to how the original Doom and Wolfenstein player models were very alike, also having been said by Tom Hall to be related somehow.

Quake Champions gave him a number of different faces, some based on his classic appearance in original doom, Doom Marine's's appearance in Doom 3, and a unique model for his Doom Slayer appearance as well. The classic "Doom" model (from Quake 3: Arena) does not show his face.

In Doom Eternal he is given yet another face (somewhat similar to Doom 3's appearance/hair color, mixed with facial model of B.J. Blazkowicz from Wolfenstein (2009).

The Art of Doom Eternal gives several facial designs that were ultimately rejected.

A life-size statue of Doom Slayer was made, a picture of which, where John Romero is standing next to it, shows the Doom Slayer's face.

In Doom (2016) and Doom Eternal, his face can also be seen in reflection of his helmet under certain conditions. In The Ancient Gods - Part 2, he removes his helmet, revealing his face before killing the Dark Lord.

Costumes and Skins
Doom Eternal introduces a number of unlockable character skins for the Doom Slayer. While the majority of the player skins can be unlocked during normal gameplay, some are only available through DLC, external promotions or the Slayer's Club. For example, the Doomicorn, Purple Pony and Night-mare skins are only available as part of a time-limited Twitch Prime promotion.

Trivia

 * "Doom Slayer" is the main official title for this character in Doom (2016), Doom Eternal, and Quake Champions and is the title used for the Slayer's Testaments.
 * The Slayer seems to be a fan of collectible toys, and is especially playful when he sees one based on himself.
 * In Doom (2016), the Slayer fist bumps his Doomguy counterpart when it is collected, a special animation not shown with any other collectible figure.
 * In Doom Eternal, he again has a special interaction with his toy counterpart. In this case, he extends his Doomblade and flicks up the arm of the toy to match it.
 * In an unused line in Doom Eternal, the Khan Maykr states that she will send Doomguy back to his "...homeworld, the Earth-world in the seventh dimension, before the time of the demons. You will live there again as before, with no memory of the suffering you have been through. All your loved ones will be there, waiting to embrace you again, as if you have never left..."
 * In Doom Eternal, a log entry by Elena Richardson states that Doomguy's blood type is AB+.
 * It was implied that he was rendered mute after Samur Maykr's gift through various codex entries in Doom Eternal, which was proven to not be true in The Ancient Gods - Part Two.
 * According to Hugo Martin, The Slayer can see hallucinations of Daisy throughout Eternal.
 * By linking a Bethesda.Net account to Doom Eternal and The Classic Doom Games, one can acquire various skins: Doom 1993 (Indigo Doom Marine), Doom 2 (Brown Doom Marine) and Doom 3 (Red Doom Marine)
 * The account link rewards resemble the doom marines from the opening graphic to Doom 1993.
 * Although stated to be officially canonical to the Doom timeline, none of the 3 Marines in either Doom 3, Resurrection of Evil or The Lost Mission are in any way related to "The Slayer". The Slayer himself is only implied to exist in recovered tablets found in the main campaign.
 * As the greatest of his foes the Doom Slayer has in recorded killed three Titans, the Dreadnought from Taras Nabad, an Titan from the Umbral Plains later the Titan's Realm, and finally the Icon of Sin (which was unrecorded to have been killed by him twice by The Ancient Gods). It's unknown how many more he could have killed but is confirmed from the Book of Kings, Slayer Testaments, and from playing the game that he has in record killed three Titans.
 * The title "Doom Marine" or "DOOM Marine" first appears as a title for the main player character appears in earlier material including the Doom RPG website, and the introduction to the Doom 3: BFG Edition manual:
 * "Step into the boots of DOOM Marine and experience his adventures for the first time, or all over again. Featuring enhanced versions of DOOM 3, DOOM 3: Resurrection of Evil and the all-new DOOM 3: The Lost Mission, plus the classics DOOM and DOOM II, DOOM 3 BFG Edition is the consummate DOOM experience." 
 * "Doom RPG is a first-person turn-based role-playing game set in the Doom universe. Developed specifically for your mobile device, you reprise the role of the Doom Marine made famous in the groundbreaking id Software titles Doom, Doom II, and Doom 3. Say goodbye to humdrum mobile gaming and prepare yourself for the return to Mars in a showdown with the legions of Hell!"
 * Doom Slayer owns a book about Daisy (presumably one he wrote himself). He also owns a biography about Flynn Taggart and plenty of comics from Taggart Comics Group. He also signs into his computer as FLYNNTAGGART, but whether or not it means anything, either a previous user of the computer or his own credentials (his name or alias), is anybody's guess...
 * He also has the Soul Cube in his collection as a nod to the Doom Marine from Doom 3.
 * A "silent protagonist" in the tradition of many fictional heroes, the Slayer does not talk until the end of the events of the video games. Flashbacks during Doom Eternal suggest that this may be a result of insanity.
 * The Slayer is considered a Dark Lord by the denizens of Hell; as he is unquestionably the most ferocious and skilled warrior to ever inhabit the realm, he holds the right to lead and rule them. The Slayer is either unaware of this fact or ignores it in his hatred for all Demon kind.
 * It's possible that "Doom Slayer" as a title refers to anyone brave enough to oppose the forces of Hell, single-handedly. Also, with the presence of an exiled Sentinel in Hell itself, this could imply the Marine wasn't the first to bare such a title.
 * Initially, the intro cinematic for the base campaign and the final cutscene of the Slayer killing Davoth in TAG 2 was not animated to take into account helmentless skins. Eventually, an update added a bonus animation to those two scenes if the player is using a skin without a helmet. For the intro cinematic, the Slayer sets his helmet down on the table instead of picking it up. For the scene when he reveals his face to Davoth, instead of him pulling off an invisible helmet, that scene is skipped completely, and he is not seen "removing" an invisible helmet.