“Servitude”
In Book VIII, 37, Malory calls it an “isle.” In VIII, 38, he calls it also a “valley” and places it in Wales.
It was ruled by the evil giant Nabon the Black. Its capital was Glait Castle.
Nabon held a tournament there to celebrate the knighting of his son. Lamorat and Tristan both attended, and the latter slew Nabon and his son. Afterwards, it was known as the Land That Tristan Freed, and was given to Sir Seguarades.
See also
Flower of Lyonesse | The Legend of King Arthur
Sources
Prose Tristan | 1230-1240
Le Morte Darthur | Sir Thomas Malory, 1469-1470