Ultima Underworld: The Stygian Abyss is a computer game developed by Blue Sky Productions (later Looking Glass Studios) and published by Origin Systems in March, 1992. It is part of the Ultima series. The game was born from a concept brought to Origin by Blue Sky of a "simulation style" RPG using advanced 3D engine technology that was later combined with the Ultima gaming universe created by Richard Garriott. It was released before and featured significantly more complex technology and gameplay than the exceedingly popular Wolfenstein 3d by id Software, but had higher system requirements.

The player is cast in the role of the Avatar, the protagonist of the Ultima series. After being drawn back to Britannia by an apparition, the Avatar witnesses the kidnapping of Baron Almric's daughter by the wizard Tyball. The wizard escapes and the Avatar is caught by the baron's guards and taken before him. The Avatar is found guilty of the crime and banished to the Great Stygian Abyss, to either rescue the baron's daughter or perish within. The player has to deal with the survivors of the failed colony and, in the end, discovers a plot to summon a daemon, the Slasher of Veils, into Britannia.