Sosaria

Originally, the world of Sosaria was actually four continents.

Lord British's Realm - ruled by Lord British and the Lost King. Later known as Britannia.

The Lands of Danger and Despair - ruled by Lord Shamino and the King of the White Dragon

The Lands of the Dark Unknown - ruled by Lord Olympus and the King of the Black Dragon

The Lands of the Feudal Lords - ruled by the lords of Castle Rondorin and Castle Barataria

After the defeat of Mondain, three of the four continents seemingly disappeared, leaving only Lord British's realm. The Lands of Danger and Despair were later rediscovered as the Serpent Isle, so it seems likely that the other two continents still exist. Ultima Online: Samurai Empire posits that the Land of the Feudal Lords was transformed into the Tokuno Islands by the cataclysm.

Lord British's realm was then referred to as "Sosaria" and was nearby the lost continent of Ambrosia.

After the defeat of Exodus, Sosaria became Britannia.