Duke Nukem 3D: 20th Anniversary World Tour

This easter egg is found in the Episode 5 level, ‘Mirage Barrage.’

From the start point, activate the cheat menu. The only cheat you need to select is ‘Give All.’

Select and turn on your jetpack and fly pretty much straight from your start point. After a few seconds, you will see a pyramid to your left. Fly towards it and you will see a squared area carved out in front of it. Land in this area and against a wall is Sam from the Serious Sam game. Walk up to him and Duke will say, “Why so serious, Sam?”, and the achievement will unlock.

Thanks to PurrPleeHaze for the video: