Lets try to straighten this shit out as people are still confusing Doomguy and Doomslayer as being 2 different people.
DOOM Summary and Theories for Faggots:
Innumerable Aeons ago, The time dysfunctional Hairball Multiverse that is Hell invades other universes, Seraphim/Nephilim (Earth Lore knows as Angels) intervene. Realms lost get absorbed by Hellscape, the rest expel demons + close portals from Hell with angels help. (Proof of Doomguy Earth's presence during war against Hell found in Doom 2016, medieval armors and weapons from around the Dark Ages/ Skyrim Easter Egg at Kadingir's Sanctum, most likely a darker demonic version of the Holy Crusades vs Hell portals)*
Nephilim/Seraphim form Argent D'Nur as a "Gate Keeper" universe to keep Hell at bay. The "Holy Order" of the Night Sentinels are formed with the
multiverse's champions (Blazkowics bloodline might be Night Sentinel reject sent back to Earth). Argent Wraiths are most likely a cadre of Nephilim Angels.
A Night Sentinel now known as the "Betrayer" bargained Argent D'Nur for son's resurrection (Son becomes "Icon of Sin")
Argent D'Nur falls, Hell runs rampant where/when-ever. Mars took the brunt of our normal-verse timeline**, sacrificing the Martian population to create Soul Cube for their "Champion" to defeat Hell. (Doom 3 Lore)
Blast forward a bit, Wolfenstein: The Spear of Destiny. Blazskowicz finds the spear of destiny, a hell portal opens and a Pre-cyberdemon comes through. BJ kills the pre-cyberdemon by cutting its arm and leg off with the spear of destiny.
Doom 1 + 2 + N64 = Doomguy's adventures, N64 = seals self in hell to fight for eternity. This means all Doom .WADS are now canon as Doomguy rips and tears in every reality Hell contains.
Doomguy kills the Titan (A gigantic Shadows of Colossus/ God of War boss sized super demon) WITHOUT SUPERPOWERS.
Seraphim take notice and grant him super powers turning him to Doomslayer lite.