Nightmare!

Hardest skill level. Nightmare not only disallows use of cheat codes, but monsters and projectiles move at double speed, and all monsters respawn a few seconds after you kill them. Nightmare is equivalent to playing the game on Ultra Violence with the -fast and -respawn command line parameters (only you can cheat that way).

Early versions of Doom did not support Nightmare mode. The initial mail-order releases were version 1.1 and only included the first four skill levels. Version 1.2 added Nightmare mode support; most shareware versions of Doom distributed were version 1.2 and so included Nightmare mode.