Slayer timeline

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This is the timeline from Doom (2016) /Doom Eternal /Doom: The Dark Ages which has links to both classic Doom and Doom 3, see shared timeline)

Background

Doom 2016's dates are based on MTC (Mars Coordinated Time) (although it doesn't appear to be correctly used as MTC doesn't calculate earth months or years (only hours, minutes and seconds), and months and years are given in the game). It's difficult in general to give Mars "months' as it's moons do not orbit in the same way the Moon orbits around Earth.

Another strange anomaly appears in that date given on the invasion maps in Doom Eternal are 18/09/2016 (which is clearly much earlier than 22nd century). Other dates in the Codex/Story Of Earth confirms earth style dates of 2151 and 2163 (assuming they aren't in the strange "MTC" system of Doom 2016).

Perhaps the system in Doom (2016) is a mix of MTC, and Earth based time, but how that works is not clear.

In real world planetary scientists track Mars years starting with Mars Year 1 (MY1) which began on April 11, 1955. MTC only tracks Martian hours, minutes, and seconds (not the year). But associating the year with Martian time implies it is separate from Earth time. For example, in real world calculations the year 2145 on earth would be year 102 on Mars if date is properly converted. While a martian date of 2145 could conceivably be in year 4107 CE/AD on Earth calendar (or the 42nd century). While a Mars year of 2149 (Doom 2016) would be year 4115 CE/AD on Earth. This is also assuming there hasn't been a calendar reset on Earth at any point (AE system used in Art books)... Which makes the two calendars overlap closer.

If the dates are intended to be based on Earth time (as the earth based months indicate) it would be in the 22nd century, but seemingly to overlap previous series that apparently take place in the 22nd century (either originally or through retcons). For example, as for when Classic DOOM series takes place in its respective universe see Doom Timeline and Doom Retcon Timeline (with original interpretation that they took place in 21st century and later retcons implying they took place in the 22nd century). However, with the placement of Doom 3 being before Doom 64 (as per the Doom 64 remaster backstory) it would suggest all the games took place in the 22nd century.

This timeline only covers the dates relevant and mentioned in the 'reboot' series (as there is no clear indication how it interacts with events of Doom 64, and when the date of that event took place, or what universe it took place in, in this reboot). Doom 64 is mentioned as a foot note (separate from the main timeline)...

While this timeline overlaps fairly well with Doom 3's timeline (see Shared timeline) they are left off this timeline for easier comparison with the Doom 3 timeline. Though Doom 3 does get a notation in the classic doom section (see also Doom Classic Bundle).

Also because there is a split in the fan community as to what type of 'reboot' Doom (2016) actually is, and how it relates to previous games, this is also discussed below.

Doom 3

Hugo Martin has gone on the record to say every numbered game is indeed canon (including Doom 3). He later clarified that everything including Doom Reboot and Doom 3 links back through Doom 64. The Doom 64 Remaster backstory to UAC Mars Research Facilities appears to be a direct reference to Doom 3 events as that was the location visited in that game.

Though there elements of reboot universe mars technology, ideas, and characters that are similar to Doom 3 technology and history there are certain things that differ. Such as vastly different energy technologies and different UAC cultural structures. With Doom 3 being far more corporate and Doom 2016 being more cult/religious like.

Even the Martian Hero and Doom Slayer while mirroring similar concepts in both universes have differences. And the history of Martians in general vs the Sentibel Hebethen history are not 100% analogous.

One theory goes...

The timeline of the DOOM series can be a bit convoluted, but basically everything up until DOOM 2016 featured Doomguy as the protagonist. DOOM 3, 1, and 2, in that order, chronicle his battle with the forces of hell. After those, DOOM 64 sees the Earth quarantine all of the UAC research installations, but a plot by the demons lures Doomguy back into hell once again. Serving as a cap on the story, at the time, DOOM 64 had Doomguy kill the Mother Demon and remain in hell to destroy demons until his last day.

Though if Doom 3 is a prequel to Doom 1, or a sequel to Doom 1-2 (Doom RPG timeline, or two events occurring parallel to each other during Doom 1 by two separate marines during the same Martian invasion, or just two interpretations of the exact same event (in the same timeline)... Has never been confirmed... There has only been speculation...

One Earth Dimension or many

While DOOM Eternal currently only refers to one Earth dimension. Some other ID software games such as Quake Arena, and Wolfenstein Young Blood do establish there are multiple alternate Earths. It's not clear if this is the case with the DOOM reboot universe (other than if Quake Champions is taken into account). DOOM Eternal's plot does seem to imply that the demons are specific to one particular Earth realm, and it might be the same one seen in every game of the DOOM series. DOOM Slayer himself has only been confirmed has having come from Earth originally, but he has not been confirmed to have originated from an alternate Earth as of yet (at least not specifically or directly). Neither has it been specific dates when, where, and how in the timeline the events of Classic DOOM/DOOM 64 games take place, other than it is part of DOOM Slayer's past. The reference to Doom 3 in the Doom 64's remastered backstory however suggests it exists in the retconned 22nd century timeline seen in Doom 3 (similar to Doom RPG universe history).

In an unused line in DOOM Eternal, 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...

Classic Doom

Date unknown 21st century in some console ports (c. 2012 in the novels, or 2022 in the SNES Doom) or 22nd century (c. 2144-2145, according to later marketing for Doom 3 and Doom RPG releases) (parallel universe, or dated to AE (Ancient Era calendar) (based on it being a sequel to Doom 3 in the Doom 64 remaster backstory)?

• Doom I Enhanced (and Doom II Enhanced and/or Doom 3?) 2019 release with a retcon to Episode 1's setting to Mars itself.
  • You're a marine--one of Earth's best--recently assigned to the Union Aerospace Corporation (UAC) research facility on Mars. When an experiment malfunctions and creates a portal to Hell, the base is overrun by blood-thirsty demons. You must shoot your way out to survive.
• Doom I + Doom II (version undefined but does include PC versions backstory that includes Daisy, rather than most console games that change the plot and leave Daisy out, consoles and novel tended to place classic games in 21st century though). Generally speaking stuff like Final Doom/Legacy of Rust are unimportant to Doom Slayer's main history, but may still have happened during his life (see Doom I + Doom II).
  • DOOM, you're a space marine, one of Earth's toughest, hardened in combat and trained for action. Three years ago you assaulted a superior officer for ordering his soldiers to fire upon civilians. He and his body cast were shipped to Pearl Harbor, while you were transferred to Mars, home of the Union Aerospace Corporation. The UAC is a multi-planetary conglomerate with radioactive waste facilities on Mars and its two moons, Phobos and Deimos. With no action for fifty million miles, your day consisted of suckin' dust and watchin' restricted flicks in the rec room.
  • Taking place on Earth, DOOM II leaves the moonbases of Mars for more familiar territory. Demons have invaded Earth, and the only force to resist them is you, fresh off of staving off the invasion of Mars. DOOM lI takes you through a space station to save the last strands of humanity, continues through Earth's abandoned and destroyed cities, and then takes you back through hell, in order to confront the source of the Earth invasion, the Icon of Sin.
  • Doom 2 comic (this is essentially a retelling of Doom 2).
• 2145
  • Doom 3 (the placement of this is nebulous, but is mentioned as being before Doom 64 in Doom 64's backstory, or overlaps Doom I Enhanced backstory). This game appears to use Earth time (not Martian years).
• Years later, after events of UAC Mars Research Facilities invasion in Doom 3, Doom 64 begins.
  • DOOM 64 is an action first-person shooter and a direct sequel to the events of DOOM II. Years have passed since you stopped Hell's invasion of Earth. Quarantined for humanity’s safety, the UAC research facilities on Mars were abandoned and forgotten...until now. A signal from a degraded satellite suggests a single entity remains, and it has the power to resurrect the demons. As the only surviving marine who fought Hell’s forces, you are sent on a lone crusade to hunt down the Mother of Demons and stop a renewed demonic invasion. Fight against demons in your crusade to hunt down the Mother of Demons and stop Hell's invasion.
• Doom 64 ends: The Doom Marine enters hell one more time and vows to fight demons for eternity to prevent them from invading earth again.
• The Lost Levels: The sister Resurrector decides to take revenge on the Marine, sending him back to Earth. He fights his way through a UAC base to find a portal back to Hell, fighting his way through the new patch of Hell he finds the Mother Demon's sister again, and takes takes revenge on her. After her defeat he again decides to remain in hell walking the perpetual path through Doom. He gets stuck traveling through time and space (traveling back thousands of years)
• Worlds and Time: The slayer remains in hell for roughly 12.8 billion years.
• He is discovered injured by the Night Sentinels who nurse him back to health and make him their Champion.

Ages

• Unspecified...
  • One Dekillion years ago (1,000,000,000,000,000,000,000,000,000,000). Rise of Davoth.
• Time of Grief
• First Age/First Age of Ascension
• Second age
• Third age
• Fourth age

2149: This appears to correspond with Doom 2016.

Doom 2016/Eternal/The Dark Ages

Note: Most of these dates are also listed with the designation "MTC". This is a reference to "Coordinated Mars Time".https://en.wikipedia.org/wiki/Timekeeping_on_Mars#Coordinated_Mars_Time_.28MTC.29 Some additional dates appears in The Art of Doom, some of these dates appear to be from earlier concepts and in some cases shifted about in the final game see below.

• 1 dekillion years ago.
• 12.8 billion years ago.
• 80 million years ago.
• 60 million years ago.
• 80,000 Years Ago
  • Doom The Dark Age

• 21st Century
  • Early; Water is discovered on Mars. Very little happens in way of colonization, except exploration over the next century.
  • Late; Advanced Rocket Launcher is developed.
• c. 2019 (assumed date)
  • Samuel Hayden is born.
• 2095
  • The Argent Fracture is discovered. Argent plasma was discovered in a trench on Mars by an expedition from the SS Amundsen.
• MTC 2096
  • The Union Aerospace Corporation established an outpost on Mars to extract Argent plasma from the Argent Fracture.
• MTC 2122
  • March 10 - A statue of the Praetor Suit was excavated (Artifact U9) from Site 3.
• MTC 2124
  • November 16 - Two Ancient Sentinel statues were excavated (Artifact J46) from Site 1.
• MTC 2127
  • The Argent Tower is completed.
  • 6 Months Later - Samuel Hayden transfers his mind over to a cyborg body to continue his existence, after his body breaks down due to Stage 4 Cancer.
• MTC 2136
  • The Advanced Research Complex is completed.
• MTC 2143
  • During one of the first teleporter explorations, the Corrax tablets are retrieved from Hell in a UAC Automated Survey.
• MTC 2145
  • 128 - The first Project Lazarus Manned Expedition is sent to Hell. The expedition ends with complete loss of human life, but achieved its mission. The sarcophagus of the Doom Slayer is recovered from Kadingir Sanctum.
• MTC 2146
  • Industrial accident related deaths in the Foundry dropped significantly thanks to new UAC safety protocols where employees exposed to dangerous materials are sent to the Lazarus Labs.
  • 351
    • The first Project Lazarus Tethering Operation is conducted.
    • Pinkies are discovered for the first time.
    • The "vade mecum" compendium is recovered from the Umbral Plains containing information on the Aranea Imperatrix.
• MTC 2147
  • 016 - The second Project Lazarus Tethering Operation is conducted. The expedition ends in an confrontation with the Hell Guard.
• MTC 2148
  • 177 - The second Project Lazarus Manned Expedition is sent to the Great Steppe. The expedition ends with similar losses. The petrified remains of a Baalgar demon is recovered.
• MTC 2149
  • c. February - The Argent Facility - UAC base had its last accident.Echo/hologram UAC facility level, would be accident free for 221 days
  • October - Doom (2016) begins. The Doom Slayer is release from his sarcophagus by Dr. Samuel Hayden and fights his way through the UAC Argent facility and Hell.
  • December - Indentured servants on the offworld Tei Tenga Mining Station were replaced with robotic workers, optimal output expected to return to normal by this date.
• MTC 2150
• MTC 2151 - The Armored Response Coalition is established by the Allied Nations to combat the Hell invasion of Earth.
• MTC 2163 - Doom Eternal begins. The Doom Slayer returns to Earth with the Fortress of Doom to quell the Hell invasion.

Misc...

• July 16: Geffroy Regional Medical Center 10th Annual Blood Drive
• 2016
  • September 18 - Map of Earth"18/09/2016" Map in Doom Eternal Codex

Art of Doom (2016)

The Art of Doom has an early version of the timeline from the game its included here for comparison. Note: The book also mentions "AE" designation along with the "MTC" designation but there is no explanation (it is used for the 1366 date, and just talks about it being in Argent Records/UAC Archival Research, and appears to related to being an era 'over two millennia ago'). Note: Two millennia = 2000 years ago.

• C. MTC 149 (AE/1366/88)- First recorded trans-dimensional encounters with demons and humans apparently on Earth. First time demonic races were documented by humans (this may also be associated with Argent D'nur?). First known transdimensional encounters occurred over two millennia ago.
• 21st Century
  • Early; Water is discovered on Mars. Very little happens in way of colonization, except exploration over the next century.
  • Some weapons like rocket launcher were designed late 21st century... The proven technology would be stocked late into modern times.
• MTC 2049 - Plans for the colonization of Mars begins. UAC, Mixom and I/O Logistics lay down the foundation of a new civilization on Mars. It wasn't until the UAC involved itself in the late twenty-first century, acquiring key personnel and territorial rights to the majority of the planets surface, that the plan finally evolved into an achievable enterprise. Over the next century UAC and MIXOM (along with third-party contract organizations, such as I/O Logistics) lay down the foundation, piece by piece, for a new civilization on the barren planet."
• MTC 2100
  • 000 - Early tethering operations firsthand discover possibility of species-demon hierarchy.
• 2115 - A region known as the Well is discovered on Mars.
• 2117 - Argent facility construction is compeleted.
• 2127 - Argent Tower project is put into production. Samuel Hayden dies six months later.
• MTC 2136 - Advanced Weaponry Division is completed.
• 2143 - UAC automated survey suggests that barons of Hell are the current Royal Guard of the dark lord of the fourth age. The nature of the entity is uknown but presumed as the Royal Guard's power has significantly increased so too has the power of their master.
• 2144 - Early manned UAC Expedition to Hell, uncovered one particularly interesting tomb adorned with powerful holding runes and unique relics. The relics were successfully returned to Lazarus Labs for processing and study, despite the expedition's loss of all human life.
• 2145 - Manned Expedition of 2415 reached the Great Streppes.
• MTC 2146 -
  • 118 - Twins demon is discovered.
  • 351 - Pinkies discovered.
  • 385
• MTC 2147
  • 016 - They recover a hell warden.
• MTC 2148
  • 139 - Doomed UAC Trans-Dimensional Expedition most of the explorers were set upon by Barons of Hell.

Notes

The Dating system appears to be loose and confusing a mix of Martian dating and earth dating, that simply are incompatible with each other. This would not be so much of a problem if it stuck strictly to one dating system or the other. But using what looks like Julian/Gregorian months with use of 'late 21st century' adds to the confusion.

Most dates are designated with the MTC but there are a few exceptions where a date is shown both with MTC and/or without (2095, 2115, 2147, etc) and/or specific months.

The one use of 385 days would definitely have to be a Mars date rather than earth date (Earth only has 365 days). Assuming this was not originally a typo, it however appears to have been 'fixed/retconned' to 351 days in the released game, placing it under the number of a year of Earth days.

Another thing of note is that is said that Hayden has "existed" for over 130 years now. His birthdate when he actually 'existed' as a human, is not known. This means his exact age is not known. This makes for some rather interesting interpretations... It's not entirely clear if means his 'existence' is over 130 from the time he became a cyborg, or that he has been a cyborg for 130 years (the 'over' includes his human history), or if its a mix of both that his birth was around 2019 (but earlier that date). If it means he has been a cyborg for 130 years or over, this would make the timeline actually possibly extend into the 130.

While there, is the date of October 2149, however its context is not entirely clear (and it's never fully explained), except that it does relate to some data being shown on screen (related to the Slayer). It could have easily be a past date left on a document that was written up earlier, rather than a 'current date'. There is a reference later to upcoming problems on Tei Tenga in Doom in December, but it doesn't actually list a date, actual year date.. If the 2149 date was for instance a datum related to 2149 some 130 years before (based on Hayden's age) it could easily have be ~2250-2279s.

If this was true, it does make for something interesting as the October 2149 date appears to coincide with data about Doom Slayer and his sarcophagus. In Doom 3 one Pierce Rogers was actually doing research on a sarcophagus of The Hero, which would put the date of discovery in Doom 3 and Doom (2016) as overlapping events (as 2145 date is something both games share in relation to the sarcophagus/hero, etc). If the date is in relation to data concerning what was found in that sarcophagus (or what it lead to), research on discovered Hero/Doom Slayer could have continued on into 2149 and beyond.

Doom Eternal clears this up however Codex/Story Of Earth, which indicates that the new game takes place in 2163, and talks about events in 2150-51 as well.

The map showing the "Remaining Human Populations - Part 1" has the date of 18/09/2016 listed on its top right corner.

The Arc Complex has advertisements for a 25th Annual Arc Expo.

Behind the scenes

Soft reboot

It was theorized that the new series was a soft reboot back when Doom (2016) came out, many theorized that Doom 64 was a direct prequel to the game (but took place in another era or parallel universe). Many believed it starred the same protagonist from all the previous games.

This was later implied within Quake Champions where Doom Slayer was said to have owned the pet rabbit Daisy at one time, and it talks about his previous armors"The left hind foot of Doom Slayer's pet rabbit, Daisy, killed by marauding demons when their invasion of Earth began. He carries it as a reminder of innocence lost, not for luck--after, all it wasn't lucky for Daisy.". It was also hinted in the game itself, in the Slayer Testaments which talked about him traveling between Worlds and Time (but it was unclear if this meant 'time on earth' and alternate earths or just worlds like Hell and Argent D'nur).

Like the game Quake Champions repeated the information from Slayer Testaments with additional information. That Doom Slayer (codenamed: Doom, B.J. Blazkowicz in some contexts) was the hero of earlier Doom games (and descendant of the heroes of Wolfenstein and Commander Keen) traveled across Worlds and Time.

Regardless of if the game takes place in the 22nd century or 4th millennium. In the games, the UAC seems to be less a industrialized corporation (with some corruption), but more of a cultic religious organization (which demanded that its employees be devout worshippers of Hell), with various ranks based on its priestly devotion (whether this was something only top officials knew about, or tricked down to basic civilians is less clear), or perhaps its something introduced into the UAC in the last few years of its existence (there is no indication of it in previous games Doom 3, Doom 1, or Doom 2, etc, except maybe a few individuals who turn out to be the enemies).

The developers have since confirmed Doom 64 being the direct prequel to Doom 2016, releasing the game (as a preorder bonus to Doom Eternal) with an extra campaign The Lost Levels which makes a reference to a quote from the Slayer Testament (to link the two together). It's also explained that Doom Slayer was once human, but he has been 'enhanced' since then.

MTC dating

It's not clear how MTC relates to earth time. One way of calculation of Mars in relation to Earth, is "My" (Mars Years) dating system... In 2017, Mars entered its '34th' year. If the dates given in the game are My, MY2149 would be extremely far in the future, roughly in the 4000s on earth (an earth year is roughly half of that of Martian year). This would technically place Doom (2016) and Doom Eternal in the far future even beyond 2145/46 dates in Doom 3, or the 21st century dates in the classic SNES Doom timeline. Essentially placing events in the 4th millennium CE (400th Century)...

Doom 3

While Hugo did confirm that Doom 3 was canon. He didn't explain in detail how or were it fit within the canon. This has lead to a number of different fan theories. Inlucing that perhaps it was in a separate universe tied into the Doom multiverse (through Doom 64/Hell) or a prequel to Classic Doom and Doom 64, or even somehow a sequel to Classic Doom and Doom II, and a prequel to Doom 64. Or perhaps somehow tied into the Slayer trilogy's universe in the Earth introduced during those games as UAC shares a number of details taken from Doom 3 (including UAC symbol), dating systems, similar character references, The Hero stelae from Doom 3, Super Turkey Puncher arcade, etc.

Though Doom 64 remaster story that many overlooked (as it only appears on a few digital game stores) makes a reference to the UAC Mars Research Facilities from Doom 3, and events of the Mars invasion during that game (but also mirrors similar story told in Doom II manual, but is less likely to be the same event due to ending of Ultimate Doom).

Below is a number of these theories discussed in detail.

Doom 3 and Doom (2016) in the same universe

One interpretation is that Slayer trilogy took place in the Doom 3 universe, but is only a 'soft-reboot' of the universe. That is to say that it takes place chronologically after Doom 3, but shares little to no direct narrative connections.

The basic layout of the timeline is almost exactly the same as that in Doom 3 (and roughly the same as in Doom RPG).

Chronologically speaking many of the backstory events mentioned overlap to very similar equivalents if not the same events as mentioned in Doom 3 (and Doom RPG). All three take place in the 22nd century.

Chronologically speaking Doom 2016 takes place 4 years after Doom 3 (and Doom RPG). Which could allow plenty of time for UAC to rebuild and restructure since the events of Doom 3. Also Mars City was a huge place in Doom 3. Not all of it was explored, and it took three or four different campaigns to explore portions of it, counting Doom RPG as well. Nor did we see any other potential landmarks/populated centers on the planet (though maps in Doom 3 could have hinted at many others). This could suggest that the base in Doom (2016) is just one of many of these various UAC bases scattered across the planet (one devoted to more occult studies, and requiring the initiates to join the cult), as opposed to more mundane industrial/irreligious institution that existed in other bases on the planet.

Furthermore there are more direct connections seen in Art of Doom (and potentially in Art of Doom Eternal) discussed in the back lore of the book. For example a reference to the region known as the Well being discovered in 2115. This appears to be taken from dates in Doom 3 (The date when the Martian Civilization was first discovered).

While ignoring the influence of time travel (spacetime and references in Quake Champions) it might be safe to see Doom 3 Marine) as obviously different character than that of Doom Slayer (despite hints given in Quake Champions and Doom RPG's website). As its one of the more confusing aspects of this timeline as it would indicate that if they are the same person they 'co-exist' roughly in the same time frame (one alive, and one suspended in the sarcophagus being tested on Mars).

Other little connections are made Doom Slayer appears to be an interpretation of the The Hero discussed in Site 3 in Doom 3. The Praetor Suit statue (Artifact U9) is part of the U-series Artifacts the same collection that Soul Cube was part of (Artifact U1). Some of the locations/backstory for the Night Sentinels mirror that of the ancient Martians (discussed in Doom 3). They even share similar names Praeleanthor and Praetor. A number of other details pointing to the Soul Cube can be seen if one looks closely within the environment throughout the levels. All seem to indicate the archaeological studies connected to the The Hero, relate to the same being known as Doom Slayer. If one looks closely these details are shown in Doom 2016 as well.

T. Washington, a minor character that appears in Doom 2016, is mentioned in Codex/Story Of Earth in Doom Eternal.

There are other little details if one looks at environments closely that tie details from Doom 2016/Doom Eternal back to Doom 3 universe.

Even minor details like the shared appearance between some monsters such as Hell Knights in Doom 3 and Doom 4/Eternal, and lesser extent the way the Imps fight in Doom 3 in both Doom 3/Eternal. Or the unused warehouse containing Turbo Turkey Puncher 3, also a game seen in Doom 3.

Developers have hinted this may all be intentional too, coming short of saying specifically if Doom 3 is or isn't part of any separate universe. In fact some of the articles at Slayer's Club seems to link many of the 'secrets' or calling them 'series lore' for the current series.

From Doom Eternal's perspective the inhabitants of Hell, Urdak and Argent D'Nur, speak only of the "Earth Realm" in a singular fashion. As if they are only fighting over one version of Earth, not several and there is only one 'Hell' which has been plagueing Earth with its monstrosities (implying that all previous Doom games had to exist within one shared universe). The Ancient Martians of Doom 3, are in fact the Hebethen Sentinel civilization of Mars. So at least from the soft-reboot perspective its not interested in considering the 'multiverse' of Earths from Quake Champions/Quake Arena series (which ties a number of champions from various earths into a place outside time and space. But that they the Outsiders are only concerned with the specific Hell, and the specific Earth. From Doom Slayer's perspective (though he may have traveled into multiple worlds including the Quake Arena/Champion tournaments), he has only been to that one Hell (not counting 'pocket universes' in control by that specific Hell), the one and only Hell which invaded his specific realm of Earth (the only earth they have fought over for centuries) (we can ignore the Cacodemon reference in the MachineGames Wolfenstein universe).

Doom 3 and Doom (2016) as parallel universes

As speculated by the fans, the reboot universe's timeline is generally considered to be a parallel universe to the original Doom, or Doom 3. Though its not exactly clear how it ties into previous series, other than Doom 64/Lost Levels ties into it, according to the developers (and Quake Champions Doom Slayer biography previously linked all of the characters from previous games together as well).

There are least two theories on these parallel universe one that classic doom takes place in one timeline, and Slayer trilogy occurs in another. Some fans believe Doom 3 was outside both of these timelines, while others argued it was part of the classic universe, or part of the Slayer universe.

In this interpretation some fans treat the new series as a 'hard reboot' disconnected from any previous series. Any objects or references to Doom 3 or Classic Doom games are only seen by the fans as 'easter eggs', having no bearing on the future or past of the Doom series as a whole.

Tied into this theory, is that Doom Reboot universe is not just any universe, might take place in the future of the MachineGames Wolfenstein Reboot universe (long after the Nazis were defeated in the 1960-1980s, but also explaining the continued existence of cult influence in business and entertainment in the world of the reboot games).

Doom (2016)'s developers and Bethesda the other hand have a tendency to shout out to these as 'Doom Lore' or "secrets". Something to look for ties into the overall series history. Even when they do call it an 'easter egg' they have a tendency to include some kind of 'lore' based explanation for why it might appear there (see Dovahkiin for an example). In their mind "It’s just another example of how great games transcend universes."

However, from Doom Eternal's perspective the inhabitants of Hell, Urdak and Argent D'Nur, speak only of the "Earth Realm" in a singular fashion. As if they are only fighting over one version of Earth (not several alternate Earths) and there is only one 'Hell' (not counting pocket universes also associated with original Hell realm) which has been plaguing Earth with its monstrosities (implying that all previous Doom games had to exist within one shared universe). The Ancient Martians of Doom 3, are in fact the Hebethen/Sentinel civilization of Mars.

There was however an unused line for Khan Makr which she offered to take the Slayer back to his Earth and let him live his life again with his family oblivous until He'll destroys his world, which suggests at least at one point developers envisioned a multiverse. Perhaps unrelated MachineGames Wolfenstein series also established idea of a multiverse of earths perhaps some were Nazis didn't win the war. In one of these universes Argent Energy was discovered according to Young Blood (tieing it into the Slayer trilogy universe).

Doom 3 in relation to Doom 64 Remaster

The backstory for Doom 64 remaster makes reference to the original events of Doom 3. That Marine was only survivee of the demonic invasion on the UAC Mars Research Facilities. However this is also similar to the Marine (Doom II)’s backstory which involved the invasion of UAC Mars Base.

Story

Years have passed since you stopped Hell’s invasion of Earth. Quarantined for humanity’s safety, the UAC research facilities on Mars were abandoned and forgotten...until now. A signal from a degraded satellite suggests a single entity remains, and it has the power to resurrect the demons. As the only surviving marine who fought Hell’s forces, you are sent on a lone crusade to hunt down the Mother of Demons and stop a renewed demonic invasion.

In addition Doom 3 is part of the Doom Classic Bundle which places Doom 3 between Doom I + Doom II and Doom 64 proper. Where the reference to UAC Mars Research Facilities is brought up both the Doom 3 and Doom 64 summaries.

Doom (2016)/Doom 3 in relation to Classic Doom

In general there appears to be much overlap for events of classic DOOM, that many fans find it hard to believe it takes place in the same universe as Reboot DOOM series (if both series and/or Doom 3 (and Doom RPG) take place in the 22nd century).

Although if classic Doom took place in the 21st century, it might roughly be possible that Classic Doom series/Doom 64, takes place in Doom (2016's past), over a century before (see Worlds and Time for discussion of time travel in the series). It would have to be assumed original military/industrial UAC evolved into the cult-like UAC over the centuries.

If Classic Doom takes place in the same universe as Doom 3 (either as a prequel in the 21st century or retconned as a sequel in the 22nd), Many fans think that Doom Reboot universe would have to be entirely separate and parallel (as it overlaps with many of the dates of Doom 3 and Doom (2016)). Along with the UAC's more cultic nature does appear at first glance to overlap with the more scientific nature of Doom 3's UAC.

Another possibility, is that instead of classic Doom taking place in 2022 (it would have to be closer to 2040 as per the Art of Doom's date for when UAC started planning Mars Base).

It seems less likely that if classic series took place in 22nd century (see Doom RPG retcons), at first glance it would overlap with the events of Doom (2016), and its history (and it seems "Our Hero'" had been hell for perpetually for a very long time according to The Lost Levels, Doom (2016), and Doom Eternal, not to mention the cryptic references to ancient Hero in Doom 3). Short of use of paradox and time travel, it would seem unlikely the events could overlap (though Doom 2016 does acknowledge time travel's involvement in the Slayer Testaments). One of the more confusing aspects begs the question: Does the Hero's counterpart co-exist at the same time the Doom Slayer is discovered?...

Regardless of which interpretations are believed, some kind of parallel universe and time travel does appear to be in place, see spacetime (if only limited the present/past of Hell, Mars, and or Argent D'Nur (not counting Quake Champions backstory in relation to Parallel earths mentioned in that series).

However, if series that are part of the Doom Classic Bundle take place in one universe, and Slayer trilogy takes place in an alternate earth, then technicality both could be overlapping parallel timelines alternate universes where similar events occurred in both, that similar parallel versions of characters exist in both. I.E. T. Washington.