Richard Garriot had the idea that since it took years to create a new engine for an Ultima game, why not use that same engine to create a ‘side story’ game while work could continue on the new engine? Thus, Worlds of Ultima were made. They used the Ultima 6 engine and were placed in very different scenarios than traditional Ultima. All of them are highly underrated.
The first one is about a pre-historic setting (think dinosaurs). The second is an altered timeline (think 19th century on Mars). And the third was going to be about Arthurian Fantasy (but the third one got canceled, alas).
There are not too many videos on Youtube about the Worlds of Ultima so I have to make do with the intros. But these were fantastic games with fantastic tunes.
Ultima: Worlds of Adventure: Savage Empire
1990
Ultima: Worlds of Adventure 2: Martian Dreams
1991