Deimos is the smaller and outermost of the two moons of the planet Mars. It is the scene of the second Doom episode, The Shores of Hell. "Deimos" is a Greek name of an ancient God, son of Ares and Aphrodite, and brother of Phobos.
Before the game, Deimos is teleported out of its orbit to float above Hell. By the time the lone Marine crossed over to Deimos, he found the human personnel there slain or transformed into zombies, and the complex was now under the command of a Cyberdemon and its minions, who were gradually transforming the advanced facilities built there by humans into a grim bastion of great evil. The base now had architecture that overlapped between the demonic style of Hell and the futuristic layout of the original man-made facility.
However, the Marine was able to battle his way to the Cyberdemon's lair at the Tower of Babel, and destroyed its infernal master, avenging the loss of human life on the moon's station.