Easter egg

Swastika
In version 1.0 of Doom, E1M4: Command Control had a swastika hidden in it as homage to Wolfenstein 3D, but it was later removed.

NIN Reference
E4M1: Hell Beneath includes the letters "NIN" (actually NIИ with a cyrillic reversed N) in a part of the level, a reference to Nine Inch Nails.

Romero's head
In MAP30: Icon of Sin, after the player teleports into the huge room with the Icon of Sin, a strange, unintelligible noise is heard. If the sound is reversed, it is revealed to say, "To win the game, you must kill me, John Romero". If the noclip cheat code is used to look behind the Icon of Sin, John Romero's severed head on a stick can be seen. Killing Romero ends the level.

Secret levels
The secret maps, MAP31: Wolfenstein and MAP32: Grosse, resemble E1M1 and E1M9 from Wolfenstein 3D. Both levels include Wolfenstein textures and SS Nazis that attack the player. In MAP31, SS Nazis take the place of soldiers and Demons are found in place of the dogs. A Cyberdemon is in the place of Hans Grosse in MAP32. In the last room in MAP32 there are four Commander Keens hanging on ropes. You must kill them to be able to exit the level.

Heretic: Shadow of the Serpent Riders
Attempting to use some of the cheat codes from Doom and Doom II in Heretic has negative effects on the player:


 * IDDQD results in instant death.
 * IDKFA takes away all the player's weapons, except the staff.

Hexen: Beyond Heretic
The first hub's secret level Bright Crucible features a puzzle involving the Heart of D'Sparil. D'Sparil was the final monster in Heretic.