Crossovers

Crossover fiction is an instance where Doom and other universes collide. Most instances of crossover fiction with Doom are fan-made. Fans have made movies and games that combine elements of Doom and other universes together.

The most famous example of crossover fiction of Doom and another universes are 2 versions of Wolfendoom which are basically vice-versa of one's graphics being used on another game engine (i.e. Wolfenstein being used with Doom). However, some may say that Doom II is the most famouse example as it has the Wolfenstin textures and monsters on 2 secret levels; and also uses Commander Keens as cameos at the end of the 2nd secret level. Incidentally, Commander Keen was the grandson of the Wolfenstein vetran BJ.

Crossover fiction is often confused with references and homages to other media, even spoofs.