Duke Nukem 3D: 20th Anniversary World Tour

This easter egg is found in the Episode 2 level, ‘Lunar Reactor.’

From the start point, activate the cheat menu. Turn ‘God Mode’ on, ‘No Clip’ off and select ‘Give All.’ It might help to turn ‘Show Map’ on as well (then press twice to get an overhead view of the level).

Open the door in front of you and walk along the corridor. Turn right and walk through the “Crew Quarters” door. Fall through the grated floor (there’s a space to do this on the left) and in one corner is a vent. Shoot it out and enter it. Follow the vent straight until the walls change color (from a darker grey to a lighter grey). Activate your jetpack then turn left and walk through the wall and you will be in a chasm.

If you have the map turned on, on your right, just before the chasm veers left into a wall, there is an alcove below you. Fly into this alcove and follow it until you see Luke hanging upside down. Walk up to him and Duke will say, “Now this is a force to be reckoned with,” and the achievement will unlock.

Thanks to PurrPleeHaze for the video: