Ultima Underworld: The Stygian Abyss

Ultima Underworld: The Stygian Abyss is a first-person role-playing video game (RPG) developed by Blue Sky Productions (later Looking Glass Studios) and published by Origin Systems. Released in March 1992, the game is set in the fantasy world of the Ultima series. It takes place inside the Great Stygian Abyss: a large, underground cave system that contains the remnants of a failed utopian civilization. The player assumes the role of the Avatar—the Ultima series' protagonist—and attempts to find and rescue a baron's kidnapped daughter.

Ultima Underworld has been cited as the first role-playing game to feature first-person action in a 3D environment, and it introduced technological innovations such as allowing the player to look up and down. Its design combines simulation elements with concepts from earlier CRPGs, including Wizardry and Dungeon Master, which led the game's designers to call it a "dungeon simulation". As such, the game is non-linear and allows for emergent gameplay.

Ultima Underworld received widespread critical acclaim and sold nearly 500,000 units. The game was later placed on numerous hall of fame lists. It influenced game developers such as Bethesda Softworks and Valve Corporation, and it was an inspiration behind the games Deus Ex and BioShock. The game has one sequel: Ultima Underworld II: Labyrinth of Worlds (1993).