Ultima VII, Part Two: Serpent Isle is the sequel to Ultima VII: The Black Gate (although its closest predecessor in Ultima's fictional chronology is Ultima Underworld II.) Serpent Isle is built on the same engine as The Black Gate and resembles a standalone expansion. Although it was originally conceived as a spinoff similar to the Worlds of Ultima games, ultimately the developers decided that the game would be an installment in the core series. However, because series creator Richard Garriott took it as a point of pride that every numbered Ultima game was built on a different game engine, Serpent Isle was given the title Ultima VII, Part Two.
The Avatar discovers that The Guardian has sent his minion Batlin to pursue Iolo's wife Gwenno into a lost world called Serpent Isle. This unexplored realm was once known as The Lands of Danger and Despair, a continent of Sosaria that was last seen during the events of Ultima I. Separated from Britannia by a great sundering, this land was later colonized by dissidents who objected to Lord British's Eight Virtues and fled from his lands to found their own cities based on different principles.
The Avatar, Iolo, Shamino and Dupre travel to Serpent Isle to stop Batlin from enacting the Guardian's plot to unleash the forces of chaos. There, they explore the cities of Fawn, Moonglow, and Monitor, each of which follows a twisted version of the Three Principles: Love, Truth, and Courage.
This game is currently available at gog.com and is included in the "Ultima 7 Complete Edition"