He ruled a fortress in the Straits of Sorelois in which many knights were imprisoned, among them La Cote Male Taile, and King Carados of Scotland. Lancelot and Brunor the Black liberated the fortress by defeating Plenorius and his brothers. They let Plenorius keep the fortress provided he pledge allegiance to Arthur.
Plenorius and his brother, Playne de Force, became champions of the Round Table on the same Pentecost as La Cote Male Taile,
and Sir Plenorius' brethren were ever knights of King Arthur.
Eventually Plenorius followed Lancelot into exile and was made Earl of Foise (Foix), but Malory seems not to tell us what happened to the other brothers.
Prose Tristan | 1230-1240
Le Morte Darthur | Sir Thomas Malory, 1469-1470