About me: The Doom Marine was originally a human from Earth who served in the Marines and was dishonorably discharged due to assaulting his commanding officer after refusing to fire upon a crowd of unarmed civilians, and was later transferred to the UAC Phobos base as security. The Marine soon found himself trying to prevent the demons of Hell from invading Earth. After having fought demonic hordes through Phobos, Deimos and Hell, he returned home to find Earth overrun by demons and his wife, child, and pet rabbit Daisy, among the billions of people killed, giving him further reason to pursue hunting the demons. He continued fighting the horde on Earth, before taking the fight to Hell once more.
Following this, the Marine was decommissioned. Some years later though, he was called upon again to fight the hellish force attacking a space station, prompting him to return to Hell where he defeated the Mother of All Demons. In order to save Earth, he decided to close the portal from inside Hell and continue the fight against the demons.
The Mother's sister, Resurrector, wanted vengeance, thus teleported him to Earth to kill him there. But he fought his way to Hell again, and defeated her there. Once more, chose to remain in Hell to fend off the demons.
At some point, he somehow became stranded on the world of Argent D'Nur, ending up in a different dimension entirely after his indefinite stay in Hell. There he is captured by the world's inhabitants, the Argenta, and brought before the Order of the Deag in Sentinel Prime. The Argenta were unable to understand his language and he was incoherent, as his mind was crippled with rage and he only talked about demons. The Priests decided to put him through a trial by combat in the Blood Arena in order to judge on whether he would gain his freedom. Eventually, the Marine (dubbed as the "Outlander" by the Argenta) proved himself to be extraordinarily skilled and earning the admiration of the Argenta.
This garnered the attention of the clergy and ruling powers of Argent D'Nur, including the Khan Maykr herself, and despite being an outlander he was taken in and trained in their military, serving to spread the teachings of the Maykr race. Inevitably, Argent D'Nur was invaded by the demons and the Marine immediately entered the fight; in which his fervor for their destruction soon caught the attention of King Novik, who deemed him worthy to join the Night Sentinels. Initially the traditionalists of the Night Sentinels were very reluctant to allow the man into their ranks, as the Order forbids from having Arena-borne prisoners or foreigners. But over time, the Outlander earned their utmost respects as he proved his skills and unwavering determination at slaying Hell's demonic legions and becoming the Order's brother-in-arms.
The Marine participated in a pivotal battle against the Dreadnought, a monstrous Titan sent to lead a total demonic assault on the heart of Argent D'Nur. As the Sentinels were nearly overwhelmed by the demons, the Marine was approached by a Maykr, known as the Seraphim, who offered his help to defeat the Dreadnought. He was brought to the Chapel of Purity and submitted to the Divinity Machine, blessing him with fierce speed and great power to match his will. Using his newfound powers, the Marine took the Crucible, a weapon held for a true Sentinel warrior king, and using it to slay the Dreadnought and planting the blade inside the beast's heart. Since then the Sentinels deemed the Marine as a chosen hero, and he gained his (in)famous title: the Doom Slayer.
The Slayer lead the Night Sentinels deeper into Hell, earning many victories and unwittingly allowing the Khan Maykr to build factories in Hell's domains to harvest Hell Essence to produce Argent Energy while endorsing the Slayer's name as the will of the Maykrs. The Slayer did not care about the polity of the Sentinels and the Maykrs, as he was too focused on destroying the demons, and consequently remaining unaware of the Maykrs' insidious secrecy of harvesting Argent Energy from mortal souls at the cost of working with the demons. After the truth of the Maykrs' betrayal was revealed, the Slayer remained loyal to the Sentinels opposed to the Khan Maykr.
After years of fighting in a civil war against both demons and the loyalist who sided with the Maykrs, an opportunity to end the war was supposedly discovered. At the behest of the Order of the Deag, the Doom Slayer accompanied the Night Sentinels to Hell in the hopes of securing a final victory against the demons. However, they realized too late that the priests had betrayed them to the Khan Maykr. The Slayer and the Night Sentinels were left stranded in Hell and endlessly fought the infinite demon hordes. One by one, the Night Sentinels were killed, save for the Doom Slayer and Valen, the latter of whom allowed the demons access to the Elemental Wraiths in exchange for his son's resurrection. When Argent D'Nur fell, Valen, now known as the Betrayer, chose to stay in Hell for the remainder of his days in self-imposed exile. The Doom Slayer, on the other hand, continued his endless killing spree.
Over time, the demons themselves grew to fear him. A Titan was chosen as Hell's champion and sent to kill the Slayer, only to be killed by him. Having failed to kill the Slayer at every attempt, the demons instead lured him into a Blood Temple before collapsing the entire structure on top of him. Trapped beneath the ruin, the Slayer was entombed within a sarcophagus and frozen in stasis within the Kadingir Sanctum. He would remain there for a extremely long time until he was found and recovered by an Union Aerospace Corporation expedition led by Dr. Samuel Hayden while procuring demon specimens and Hell artifacts. Hayden led his forces in defeating the demons that fiercely guarded the sarcophagus and taking the Slayer to the UAC facility on Mars. Upon identifying the Slayer and his armor from within the Helix Stone, and the developing suspicion of his employee and former protégé Olivia Pierce, Samuel had the sarcophagus hidden in a concealed location within the UAC facility. Upon suspecting Olivia's pact with Hell and her plans for the Mars facility, Samuel intended to release the Doom Slayer in the event of the UAC losing control of its demonic experiments.