Final Doom

From Doom - NeoWiki

"Final Doom" was released in 1996, two years after the release of Doom II on the PC. It was originally launched on the PC as well as a 32 level expansion pack for Doom II containing two new episodes set in the Doom universe, "TNT: Evilution" and "The Plutonia Experiment". Both episodes were originally created by fans of the games using the game's level editing tools, then later finalized by id Software to be released in Final Doom.

"TNT: Evilution" took place on the moons of Jupiter. Suddenly, a mysterious ship appears above Io. Mistaken for a resupply convoy, it opens it's doors and covers the UAC teleportation base there in demons. Everyone is slaughtered or zombified, except of course, the player, who vows revenge, intent on destroying the source of the invasion.