The doom guy is an easter egg/achievement in Duke Nukem 3D: Megaton Edition. He can be found only in the 'church part' of the Death Row level.

It's actually really easy to activate this easter egg. First what you need to do is get to the 'altar' of the church and press e (use button) on it.









Upon pressing your use button in front of the altar, the church will transform into a satanic church with an upside down cross instead of a normal cross. Also, the light will be red, and a few assault troopers will spawn there.









Then, also, this the left 'window' will open up , you will need to go there. When you go into that room, an octabrain will apear there, kill him, and then go to the other side of the room.

After you kill the octabrain, you need to go to the other side of the room, there you will find a machine gun and the dead Doom guy. Duke will then say ''that's one doomed space marine'' and that's it, you got the achievement, you discovered an easter egg!