Sigil timeline

The Sigil timeline is the structured events of the original series of Doom games from the original 1993 release to the 2019 release of Sigil.

Sigil is John Romero's 'fifth episode' (mod) to the Original Doom. However, is is considered unofficial to the original canon  (as it does not appear to be endorsed, published nor owned by Bethesda ). It also takes some liberties with the storylines presented in previous 4th episode (Thy Flesh Consumed) and official Doom Strategy Guide. Noteably, whereas Episode 4 was supposed to have taken place in a city on earth during initial invasion, Sigil posits that it was actually some kind of limbo or purgatory before reaching earth. That after defeating the mastermind in Episode 4, the Marine attempted to catch a portal to earth, but is sent into a deeper section of Hell, one ruled by Satan, and had to fight his way out to find another way back to Earth.

Sigil was not officially released by Bethesda, but it can be downloaded as an add-on to use with the newly released versions of Doom on Bethesda's website. And it has been added along with other unofficial add-ons to recent console ports as well. Betheda's Slayer's Club refers to it as 'revisionist history', and as 'unofficial sequel'.

Background
Noteably, Doom (and the early sequels) does not have clear dates for when events take place. It is implied to take place during early 21st century in a neo-1990s retrofuturistic setting, and there are only a few references to amount of time between games. Only the SNES manual gives any 'clear dates' for the events leading upto the events of the original Doom. Otherwise there really isn't much of a timeline to actually list.

Tom Hall has indicated that the protagonist of Doom (the original Doom/Ultimate Doom which he worked on) is the descendent of the main characters from Wolfenstein and Commander Keen series. He is the son or grandson of Commander Keen (which was part of the early concepts of the ID multiverse). Tom Hall's comments that he is the son of Commander Keen, this supports the theory that the timeline of the SNES manual is accurate. Since Keen grew up in the late 1980s-early 90s and likely would have had a son in the early 21st century. The Doom RPG series also took this route with the characters B.J. Blazkowicz III and Stan Blazkowicz (though clearly from a later century). The novels which lie outside of the game canon (or canons) also places the general events in the early to mid 21st century, see Doom novels timeline.

Certain aspects about classic Doom games and their manuals are confusing and don't always seem to line up. This has opened up theories that there are events not covered in the game's or even multiple Doom Marines involved. For example Doom's original ending and Ultimate Doom's ending are different than the prologue to Doom 2 with the heroes reaching Earth by different means. Doom 2 also talks about Mars Base being invaded as well from the Outside, and the invasion stopped by the marine who lands on earth via drop ship discovering that Earth has been invaded. Some fans think this might even be the events shown in Doom 3.

Several of the later manuals and websites claim it's the same hero in all the games confusing matters further.

Note: Technically no exact date is given for Doom II, but its assumed by the fans to take place directly after Doom/Ultimate Doom at least chronologically.

Classic Doom (Original Doom Games)
(Includes Ultimate Doom, Doom II (Master Levels & No Rest for the Living), Final Doom, and Doom 64.
 * 2018 - The military, UAC's biggest supplier, has used the remote facilities on Phobos and Deimos to conduct various secret projects, including research on inter-dimensional space travel.
 * 2019 - The Marine assaults a superior officer, and is transferred to Mars, home of the Union Aerospace Corporation.
 * 2021 - July 1 - DOOMNL 01-D19305-1C is published.
 * 2022
 * March 15 - Mars receives a message from Phobos base stating that the base is under attack and requesting backup.
 * A few hours later, the marines land and try to stop the invasion. The Marine is ordered to stay behind and protect the perimeter. The marine forces are wiped out or turned into monsters. Planetary authorities send the final Marine the new DOOMNL 01-D19505-1D replacing earlier manual (now including briefing for the Marine, and images from the invasion captured by remote video cameras. The video data was computer analyzed by the UAC and the results are provided "here and only here") . The Marine leaves the escape pod vowing to take down as much of the invasion force as possible before death.
 * Knee Deep In the Dead begins as the Marine arrives in the Hangar Atrium.
 * Knee Deep in the Dead ends as the Doom marine is ambushed by the forces of hell upon arriving on Deimos, and presumably dies.
 * Shores of Hell begins as the Doom Marine finds himself alive somehow, and pushes against the forces of Hell.
 * Shores of Hell ends as the Doom Marine rappels down the surface of Deimos, discovering that it is now orbiting above Hell itself.
 * Inferno begins as the Doom Marine arrives on the surface of Hell.
 * Inferno ends with the Doom Marine killing the Spider Mastermind, the general of the Phobos and Deimos invasions, and 'rewarded' with a gateway to Earth discovering Earth has been invaded and his pet Rabbit is killed.
 * Thy Flesh Consumed begins (an interquel of sorts inserted after Shores of Hell and before or during Doom II). The Doom Marine continues to fight in revenge for his dead pet Daisy stopping the Spider Mastermind's invasion force 'before' it is sent to earth. But he is unable to prevent Hell on Earth.
 * SIGIL begins (the interquel continues) After killing the Spiderdemon the Marine's next stop is Earth — he must save it from hellspawn that has causing unimaginable carnage. But Baphomet glitched the final teleporter with his hidden sigil whose eldritch power brings the marine to even darker shores of Hell. He fights through this stygian pocket of evil to confront the ultimate harbingers of Satan, then finally return to become Earth's savior.
 * Meanwhile, a Marine stops a demonic invasion on the UAC Mars Base on Mars over the course of a few days. He is given leave to return to Earth. He was the only survivor on Mars. He returns to earth in a drop pod only to discover it has been invaded by the Demons from the Outside.
 * Doom II begins
 * No Rest for the Living begins
 * The Doom Marine continues his battles on Earth after Hell's forces were seemingly vanquished, and is forced to travel to a pocket universe to stop another demonic threat.
 * The Union Aerospace Corporation is entirely reformed, as the original stockholders, trustees and management were killed in the invasion of Earth, and begin managing two projects; one in an unknown jungle, and the other on one of the moons of Jupiter.
 * TNT Evilution begins with the Doom Marine, the last surviving marine Jupiter moon base, once again fighting against the forces of Hell.
 * TNT Evilution ends with the Doom Marine destroying the Demon-spitter
 * The Quantum Accelerators succeed in their maiden test, however fail in their second activation leaving one Gate left open for the forces of Hell to
 * 2023
 * Yearly supply ship reaches Jupiter's moon. Demons use it to invade the UAC base there.
 * The Plutonia Experiment begins as the Doom Marine fights against the forces of Hell again after arriving via pickup truck.
 * The Plutonia Experiment ends with the Doom Marine destroying the Gatekeeper, swallowing up the final demons and the accelerator.
 * The fatigued Doom Marine is decommissioned, and the military DOOM Episodes are classified. The UAC's bases are sealed, and quarantined by radioactive bombardment.
 * Years Later
 * Mother Demon survived in radiation of bombarded planetary space installation. Shielded in the radiation it resurrected the demons mutrinf them into stronger and more vicouous forms.
 * Doom 64 begins. The Doom Marine is re-commissioned to stop and the latest invasion, he stops the Motherdemon's forces which had been building in the former battlegrounds he failed to clear.
 * Doom 64 ends. The Doom Marine decides to stay in Hell to prevent the rise of another demon lord after destroying the Motherdemon.