Modders have done the ultimate Doom challenge: getting Doom running... inside of Doom!
"Yes, but can you run Doom on it?" is the eternal question of the modding community, who have endeavored to port iD Software's classic shooter to pretty much every device imaginable, from Raspberry PIs, to ATMs to mother-flipping printers. But now, one ambitious modder has topped them all, by porting Doom into an in-game Doom arcade machine. That's right, he has gotten Doom to run... inside of Doom.
Technically, the feat is accomplished with GZDoom, which is in of itself a heavily modified Doom source port that was first released in 2005. The mod in question is called "Classic Arcade", and brings with it semi-complete versions of both Doom and Doom-predecessor Wolfenstein 3D into the game. While the video shows that the only weapon available is the pistol, you still have to admire the insane amount of pointless effort involved in pulling something like this off.
At this stage, the only way anyone can top "The Zombie Killer" Doom-in-Doom mod is to actually somehow make another version of Doom playable from inside "Classic Arcade's" Doom arcade machine.
We need to go deeper!
Source: Ars Technica