Duke Nukem 3D: 20th Anniversary World Tour

This easter egg is found in the Episode 1 level, ‘Death Row.’

From the start point, walk forward to escape the electric chair. Then activate the cheat menu. Turn ‘God Mode’ on and ‘No Clip’ off.

Walk forward through the red curtain and through the doorway on the right. Turn left into a hallway, continue walking and turn left again into a small chapel. Walk towards the altar and press on the grey square and the room will turn red. The cross at the back of the altar will morph so you can walk through it.

Once you walk through the cross, turn right and walk down a passage (there is an Octabrain to the left) and you will see the Doomguy in his death throws in front of sacrificial altar. Walk up to him and Duke will say, “Hmm, that’s one doomed space marine,” and the achievement will unlock.

Thanks to PurrPleeHaze for the video: