The first of two ground-breaking Ultima Underworld games made by Looking Glass Technologies (formerly known as Blue Sky Productions) and Origin Systems.
The Stygian Abyss, first released in 1992, is one of the first Role-Playing Games to feature first-person action in a 3D environment.
Back then, these games were a giant technological leap forward for the genre, although nowadays the effect is not quite so impressive. That said: play Ultima Underworld now and you will still find plenty of joy in the gameplay. Not to mention: a serious challenge worth undertaking. The Stygian Abyss was followed by the even better Labyrinth of Worlds in 1993. Both games are still available to buy via GOG.com for very little money and are well worth a purchase.
Back in 2002 I wrote an article about the series for PC Zone magazine, called Games That Changed The World, and interviewed programmers Paul Neurath and Doug Church, and Producer Warren Spector. Here are the interviews, if you want to read more. Note to Wikipedia editors: get your facts right. These interviews, conducted by myself, were NOT commissioned or published by C&VG magazine (as you incorrectly state), but PC Zone magazine!