The "Worlds of Ultima" games are an oft-forgotten artifact of the popular Ultima series of CRPG's. They have been omitted from not only most third-party lists of Ultima games (even those that include the Ultima Underworld games), but from Origin's anthologies of the series as well. Since they were originally released on floppy disks, this makes them the only Ultima games for PC that have never been published on CD-ROM.

The series consists of two games, The Savage Empire and Martian Dreams, published in 1991. They use the same engine as Ultima VI, but are not set in Britannia. The Savage Empire is set in an alternate universe where dinosaurs coexist with primitive humans. Martian Dreams concerns an expedition to Mars during the Victorian era, but (oddly enough) is not supposed to be set in an alternate universe.

At this point, you may well be saying "Well, of course they aren't considered part of the Ultima series. They're clearly unrelated to Ultima's continuity." But that's not so. The hero of both games is specifically the Avatar from the Ultima games, and the games make frequent references to things and events from the Ultima worlds - for example, the Avatar encounters what could be alternate-universe versions of his companions in both games, and the Shadowlords from Ultima V show up in Martian Dreams. There's even a small reference to the events of Martian Dreams in the dreamworld of The Serpent Isle.