The Doom Slayer

The story begins in the distant future. Space travel is pretty much down pat. The hero — who, right now, is best referred to as the Marine — is, well, a Marine. Tough as nails, Semper Fi, gung-ho, the whole nine. Things go awry for the poor man's career, however, when he assaults a superior officer for ordering his men to fire on innocent civilians. The USMC ships the Marine off to Mars to be bored out of his skull as a glorified security guard for the United Aerospace Corporation, a conglomerate that has its dirty little fingers in every pie. That includes secret experiments into inter-dimensional teleportation technology on Mars' moons, Phobos and Deimos. The experiments aren't going well even at the best of times, with the people who pass through the two working portals on each moon emerging physically or mentally broken.  One day, the situation gets exponentially worse when Mars receives a distress call from Phobos stating, "Something fraggin' evil is coming out of the Gateways." That's exacerbated by the fact Deimos just straight up vanishes from the sky. The Marine and his buddies check out Phobos, with the Marine left guarding the perimeter. When the radio eventually goes silent, the Marine has no choice but to take a pistol and see what's going on for himself. No pressure, though. "This explains the lore of Doom please don't sue me"