His story begins prior to Doom I. He was a soldier at war that killed his commander after the commander became corrupt and told his men to fire on innocent civilians. As a result, he was sent up to the Union Aerospace Corporation (UAC) facility on Phobos, damned to spend his life as a guard doing nothing at a facility far from home.
Things got bad not long after, though. The UAC was experimenting with inter-dimensional space travel via portals. This means they would throw an object through the portal, which would carry it through another dimension, and finally through another portal that would bring it to its destination. Whenever humans would go through these portals however, they'd go insane and explode. This ramped up the portals sucking up Deimos, one of Mars' moons, and alien evils coming out of the portals.
This was when they had opened up portals to Hell, and when the demons got to Phobos and started killing his allies, Doomguy was forced to confront the demonic invaders head on in the military episode codenamed "DOOM". With his arsenal he fought through hordes of demons and killed the Spiderdemon, the mastermind behind the invasion. With his work done, he made it back home to Earth.
As soon as he got back to Earth however, the planet had been overrun by demonkind. Finding his pet rabbit dead, Doomguy's fury awoke. There were large spaceships containing humans trying to flee to space, but the demons were preventing this directly from the ports. In a last ditch effort to stop them, Doomguy and other soldiers rushed the demon-infested ports and fought to ensure humanity didn't go extinct. He made his way to the ultimate leader of the invasion, the Icon of Sin, and he fired rockets into its brain until at last it fell.
When Earth was at last saved, he had to once more return to action, as the satellites from Phobos were reporting demonic energy signatures like never before. The carcasses of the demons he had once killed were resurrected by an unknown powerful demon, and it was up to him to destroy it for good. Once he'd defeat this last foe, demonkind would be stopped forever.
Doomguy fought his way through Phobos, into Hell, and then to his last hit on the list: the Mother of All Demons. He killed her and bathed in the blood, looking to the skies of Hell in defiance. He knew however, that as long as there would be Hell, there would be demons. As such, he brandished his firearms and made the ultimate heroic sacrifice of all times; he vowed to stay in Hell forever, perpetually fighting demons, so that their kind may never ruin another world.
Several millenia later he continued fighting in his crusade on Hell, at which point he was captured by the Sentinels, a race of viking-esque knights from the far away planet Argent D'Nur. They submitted him to their gladiator arena as part of their customs, and rather than falling at the hands of another warrior, Doomguy cried out "Rip and tear!" as he tore through all of his opposition.
When demons began assaulting Argent D'Nur itself, they turned to Doomguy, a soldier fit to be in the Sentinel military known as the Night Sentinels. They trained him, and his crippling bloodlust for demons became a controlled one. He returned to the void over and over, and each time he returned a better fighter. When the Maykrs themselves turned on the Sentinels, Doomguy committed treason and received assistance from Samur Maykr, the advisor Maykr to the Khan Maykr.
Samur Maykr, also known as the Seraphim, blessed Doomguy by submitting him to the Divinity Machine. Doomguy came out as a god, a force of nature embodying Doom itself. He became a Sentinel god-king, and his eyes burned with Maykr Magic. It was on this day that a new enemy was created to every force in all worlds threatening the sovereignty of a species fighting for its life; and that enemy was the unyielding, the incorruptible, and the unstoppable Doom Slayer.
He set out against the demons like never before, and they began losing their own war. With the Doom Slayer on their side, along with their gods, the Elemental Wraiths, the Sentinels would have no chance of falling in combat... until the Betrayer came.
The Betrayer was a Night Sentinel named Valen. He fought alongside the Doom Slayer and the other warriors, but his mind wasn't strong enough to resist his own temptations. The Hell Priests gave him nightmarish visions of his son being tortured in Hell for all eternity for his sins, and in a final effort to free him, Valen gave the keys to the Elemental Sepulcher to the Hell Priests. They used this to access the Elemental Wraiths, and upon doing so they took them to Hell, where they would be tortured for all eternity so that their power might be used to produce Argent Energy: the ultimate weapon of the Sentinels' foes.
When the Doom Slayer and other Night Sentinels took the battle to the heart of Hell, they sought to destroy Nekravol. As they entered the portals, however, the Hell Priests in their trickery manipulated them so that the Night Sentinels would be separated across Hell.
Without assistance from the Wraiths the remaining Night Sentinels were doomed. They fell at the hands of demonkind, but not without a fight. Some were found with demon guts littering the area; others were found with their weapons still embedded in their devil foes. Due to the civil war, the betrayal of the Maykrs, the work of Valen and the manipulation of the Hell Priests, the Sentinel race fell to Hell's power, and Argent D'Nur with it. Even the Slayer was tricked into a trap by the Hell Priests, as he was lured into a demonic temple that was brought down upon him. After it collapsed on him, he was knocked unconscious, and because of this he was placed within a tomb locked with powerful holding runes and imprisonment cantations.
Several millenia later, the Doom Slayer would awake on Mars, his sarcophagus opened and his body free. This was the work of Samuel Hayden, one of the few survivors of the latest UAC raid on Mars. The UAC had opened portals to mine Hell for its energy, and because of this the demons could come through and attack. Olivia Pierce, one of the head administrators at the UAC at the time of attack grew close to the demons. She wanted to sit in Hell alongside her gods, and become the ultimate Aranea Imperatrix: the Spider Mastermind that Doomguy had killed long ago.
The Doom Slayer ventured into the Titans' realm and grabbed the Crucible, a weapon of great vulnerability to the demons. He found the Elemental Wraiths being tortured in Hell, and he finally put their suffering to an end by shutting this torture off and closing the Well, the ultimate source of energy for the demons (at the time). He ventured into the pits of the Titan's Realm, and there he fought Olivia Pierce transformed into the great demon known as the Spider Mastermind.
When the Doom Slayer returned to Mars, the UAC director Samuel Hayden took the Crucible from him. He claimed that Argent Energy could be used to further the growth of humanity, and he told the Slayer that he wouldn't let him ruin the progress they were making. He dimensionally tethered the Slayer into the Kadingir Sanctum, one of the many voids of Hell, and went on his way.
With the help of VEGA the Doom Slayer escaped Hell and made it to a barren Sentinel spaceship known as the Fortress of Doom. There he would be able to workshop new weapons and rest between missions. While he was gone, the cycle of history repeated itself and demons invaded Earth once again. This time humanity didn't have a chance to escape: most of its population was wiped out. The Hell Priests with assistance from the Khan Maykr were sieging the blue-green world, and it was up to the Slayer to get revenge on those that had wronged him so long ago.
First he regained all of his lost weaponry, as was part of Sentinel tradition. Then he traveled across Earth and Argent D'Nur, killing each of the Hell Priests. When he was finished with them, he traveled into Hell and destroyed Nekravol as he had planned to with the Night Sentinels ages ago. He traveled into Urdak and killed the Khan Maykr directly, and when the Maykrs had resurrected the Icon of Sin he took it upon himself to kill it too.
When the Icon of Sin was awoken in Urdak, it broke the seal preventing demons from entering it and corrupting it. Demons invaded it after the Khan Maykr fell, the Maykrs corrupted and the landscape dying. In order to prevent this, the Slayer would have to get rid of the demons in spectacular fashion. As such, he had to seek out the creator of all Maykrs; the Father.
In a twist of events, Samuel Hayden was revealed to be Samur Maykr, the Seraphim that aided the Slayer so long ago. VEGA was the consciousness of the Father imbued in machinery. The two had been hiding from demonic threats on Earth, with Samur being the Father's most trusted ally meant to protect him. The Slayer assisted in transferring Samur's consciousness from the Samuel body to the Seraphim body, and with his help set out to the Blood Swamps of Hell to retrieve the Father.
He was told by Samur to retrieve the Father's life sphere and use it to restore Urdak, protecting it and letting the Father get back in control. Instead, the Slayer destroyed the Father's life sphere, making the Father nothing more than a presence. He took the life sphere of the Dark Lord, the true ruler of Hell, and took it to the Luminarum in Urdak to use it. There he would attempt to summon the Dark Lord and kill him once and for all, destroying all of demonkind's efforts for good.
Right before he reached the summoning room Samur Maykr, overcome with demonic corruption and anger against the Slayer for his trespasses against the Father, transformed into a demonic entity and attacked the Slayer. He was defeated, but before he could be killed the Father transported him away to protect him. It was the Slayer's own battle then, no longer one against the Maykr-beings from before.
As he reached the summoning room, two seraphs aided him in using the Dark Lord's sphere to summon the figurehead of Hell's evil. Before him was himself, a dark Slayer with crimson eyes and a tattooed frame. It was him, as he was the only one fierce enough to lead demonkind: the Slayer in the Hell realm.
This evil Slayer was Davoth, a god-like being known as a Primeval. Primevals were being so powerful that there could only be one Primeval per realm, and as such Davoth was the Primeval of Hell, whereas the Slayer was the Primeval of the Earth realm. Davoth was created to steward Hell, and eventually he desired immortality for his people as he loved them and didn't want their death. Davoth became obsessed with this goal and turned on the Father, doing all he could to gain this privelege for his people.
In these ancient times the Father extracted Davoth's life sphere. From there Davoth had been guiding the forces of Hell for over a decillion years. How the Slayer will attempt to defeat Davoth from this point is unknown.
