When DOOM returned in 2016, it seemed to be hinted that the Doomslayer was actually the same character as the original game's Doomguy. It wasn't until 2020's DOOM Eternal that game director Hugo Martin confirmed that each game (with the exception of DOOM 3) is canon to his story.
The same character journeys through each title through alternate universes and worlds. The end result is a character dating back to 1993 for an epic journey that literally took him to hell and back.
The Doomguy was originally a U.S. Marine, sent to the UAC Mars Facility as punishment for punching his superior officer after he ordered him to fire on unarmed civilians. While stationed on Mars, the UAC's nefarious experiments resulted in the first demonic invasion. While attempting to master interdimensional travel using alien technology, they inadvertently opened a portal to hell itself.
Doomguy and his squad were dispatched, and the hero was left to guard the exterior of the base with only a pistol. When none of his comrades returned, Doomguy took it upon himself to enter the base alone and pick up where they left off.
From here, the Doomguy embarks on his original, iconic journey. He battles through the moon of Phobos and arrives on Deimos, which has been infected by hell itself. The hero tears his way into hell, killing every demon in his path before slaughtering the Spider Mastermind who orchestrated the invasion.
After his victory, he returns to Earth, only to find it engulfed in flames and death - Hell had invaded his home. DOOM II saw the hero defending the planet, helping survivors escape into space before once again stepping into hell to kill the demon responsible for the hellspawn - The Icon of Sin.
After Doomguy's successes, the UAC's research installations were all quarantined. The bases and labs were filled with immense levels of radiation, and for years stood entirely motionless. That was until one of the satellites monitoring the installations sent a message indicating that a single demon had managed to resurrect its spawn, turning the carnage left behind by Doomguy into new demons. In DOOM 64, Doomguy is sent in alone to deal with the new threat, doing so once again by entering hell.
After the things he had experienced and the trauma he had suffered, Doomguy decided to remain in hell to battle the demons forever and ensure no further invasion could occur. He would battle the demons for many years before winding up stranded on Argent D'Nur, the home of the Maykrs.
The Argenta, residents of Agent D'Nur, were unable to understand his language. They subjected him to their traditional trial by combat, where he defeated all challengers and earned the Argenta's respect. The Khan Maykr and the ruling class of the planet deemed him worthy of joining the Night Sentinals when he defended the planet from a demonic invasion. The other Night Sentinals were reluctant to accept him as outlanders were not permitted within their ranks but grew to admire him.
When hell invaded the planet again, a Maykr known as the Seraphim granted Doomguy superhuman power through the Divinity Machine, allowing him to defeat hell's forces. He used a crucible blade to impale a colossal demon, winning the battle. From that day on, the Night Sentinals dubbed him The Doomslayer. He would fight on with the Sentinals, invading hell and unknowingly allowing the Khan Maykr to co-operate with the demons to build factories that turned souls into argent energy. This revelation sparked a war between the Sentinals and the Maykrs.
The Sentinals would be defeated by hell and the Maykrs, and the Doomslayer alone would remain to fight on. His path of destruction caused the demons to fear him. Unable to kill him, they lured him to a blood temple, where he was entombed within a sarcophagus. He would remain frozen in stasis for years until an alternate universe UAC expedition led by Samuel Hayden discovered the sarcophagus and returned it to Mars.
In DOOM 2016, when Olivia Pierce initiates a new demonic invasion, Hayden awakens the Slayer to battle the hordes. He defeats the demons and Pierce before being betrayed by Hayden, who teleports him to an unknown location. When Earth is invaded again in DOOM Eternal, the Slayer returns to fight. He kills the Khan Maykr, as well as a new Icon of Sin before setting out to kill The Dark Lord, creator of all things. The Doomslayer again succeeds but is once again imprisoned by the Maykrs - until he is needed again.