The second of two ground-breaking Ultima Underworld games made by Blue Sky Productions (later to become Looking Glass Technologies) and Origin Systems.
Labyrinth of Worlds, first released in 1993, again features a very early (and successful) attempt to create a Role-Playing Game with a fully-explorable 3D environment. Both this – and its predecessor, The Stygian Abyss – were developed concurrently, and were a significant leap forward in terms of software technology at the time.
Both games are still available to buy (for a paltry amount) from GOG.com, and are well worth the investment, in both time and money.
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.