Ultima Underworld games both developed by Looking Glass Studios (or 'Blue Sky Productions' as they were called in the early years). The Underworld engine wasn't developed particularly for the Ultima brand, but once they struck a deal with Origin to publish it, it was decided to use the Ultima brand to garner more interest.
The last game from the 'regular' ultima series is Ultima IX: Ascension, it was the first of that series that featured a full 3D explorable world. At release it was really buggy, but some serious patching made it very playable. Though it still gets grief from fans who are disappointed that the game doesn't strictly fit the stories of earlier titles, if you don't care much about that, it is actually a great game.
Like Arx and the Gothic series, both Ultima Underworlds -and also Ultima IX- are non-linear and allow for some great exploration, sometimes into areas you're not ready to deal with yet.. creating a great feeling of a continuous world waiting to be explored.
Last edited by addink; 19th Jan 2011 at 20:15.