The Demonic Narrator commonly references "the ages" in his testaments, these ages are most likely periods of time that occured under a certain demonic leader, or demonic family line.
Before The First Age
Before the First Age there used to be a civilization of humanoids that lived in an unknown dimension, planet or universe. The civilization worshiped the Elemental Wraiths of Argent D'nur that were considered as Gods. The people of Argent D'nur feared the Wraiths and were believed to perform sacrifices in order to appease them. These Gods were protected by Argent D'nur's fiercest warriors: The Night Sentinels.
During The First Age
In the First Age, Hell invaded the civilization, but failed to conquer Argent D'nur due to the Night Sentinels' fierce defense using of the Elemental Wraiths' powers. But a lower hell priest called Deag Grav struck a deal with a warrior of Argent D'nur known only as the Betrayer, that he would return the Betrayer's son (who was lost on the battlefield) back to him if the Betrayer took the priest to the Wraiths' location. And in their sleep, the hell priest and his cabal cast a curse upon the Wraiths, trapping their souls in the process, whose power became the source of The Well (Which eventually became the UAC's source of Argent Energy).
When the Wraiths fell, Argent D'nur was defenseless and soon swiftly conquered by Hell, which accelerated the integration of the dimension into Hell's. As a reward, the Betrayer was given back his son, but he turned into Demon form; The Icon of Sin. His revenge was swift and merciless, repaying the Betrayer's treachery with suffering.
The Doom Slayer eventually appeared in Hell (whether during or after), wearing the crown of the Night Sentinels, and scoured the Umbral Plains as he went on a rampage against any Demon he came across in his quest for vengeance against the Dark Lords who had "wronged him". And on his crusade of revenge, the Seraphim bestowed upon him terrible power and speed, after which he proceeded to crush the obsidian pillars of the Blood Temples and sought to destroy the beasts of the nine circle. "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 bestowed upon the Marine, an armor "wrought in the forges of Hell, impenetrable and unyielding" (however all depictions of the Doom Slayer show him in the armor, suggesting that the wretch only modified or 'reforged' the armor rather than outright giving it too him)
And as despair spread among the Hordes of Hell, a colossal Demon known as the Titan rose to face the Doom Slayer, and a mighty battle was fought between them. The Titan fought with all the fury of his fallen brethren, but fell nonetheless to the Doom Slayer, and the Hordes of Hell were routed. Yet the Hell Walker's thirst for revenge was insatiable, and he sought the tombs of the Blood temples again looking for prey. There the "Demon priests of the Blood Temples laid a trap to capture this scourge of Hell", and as the Doom Marine, blinded by his fervor, was lured into the trap, the priests brought down the temple upon him. They entombed the Doom Slayer "in the cursed sarcophagus", separating him from his armor and trapping it in a separate stone slab. A sign was burned on his crypt, the mark of the Doom Slayer, warning all the denizens of Hell that the terror held within must never be freed.
After The Fourth Age
In the Fourth Age (nothing is known about the second and third age) the tomb was retrieved from Hell by Samuel Hayden as a safety measure in case Mars gets invaded by Hell. The Events of Doom (2016) occur during this period.
- The Doom Marine's Background is told from the side of the Demons, however much information seems to be contradictory, for example even though it is said that the Doom Slayer's armor was wrought in the fires of hell, it shows english text, company names, product codes, and even a possible military insignia, along with all of the Doom Marine's statues, and depictions from the demons being in the Praetor Suit, all before it's actual appearence.
- However it's possible to suggest because how the Doom Marine appeared sometime after the Argent D'Nur, it's entirely possible to suggest that Doom Marine is actually unrelated (apart from a group of remnant Night Sentinels that fought with him during his rampage with hell) to the Argent D'Nur