Brian des Illes, Brian de les Isles, Brien
Lord of the Castle of Pendragon, Sir Brian was
a noble man and a great enemy unto King Arthur.
He collected knights and ladies as his prisoners, among them La Cote Male Taile. Lancelot delivered the prisoners, fighting Brian until the latter yielded rather than be killed. Learning who it was that had defeated him,
Sir Brian [was] full glad, and so was his lady, and all his knights, that such a man should win them.
Lancelot departed with La Cote Male Taile and his damsel on other adventures. Possibly Sir Brian made a show of changing his ways and was left for a time in charge of his castle, However,
as Sir Lancelot came by the Castle of Pendragon [on his return to Arthur's court] there he put Sir Brian de les Isles from his lands, for cause he would never be withhold with King Arthur; and all that Castle of Pendragon and all the lands thereof he gave to Sir La Cote Male Taile.
This may be the same Sir Brian of the Isles whom Malory introduces, much earlier, as sworn brother to Nimue’s cousin Sir Meliot de Logres, and who appears again at the tournament at Dame Lyonors’ Castle Dangerous, fighting against Arthur’s side.
In Perlesvaus, the ruler of Brittany and an enemy of Arthur. He harbored Kay when Kay defected from Arthur’s court, and also joined forces with Meliant, an enemy of Lancelot’s. The three knights invaded Britain while Arthur was on a Pilgrimage to the Grail Castle. Pillaging and burning, Brian marched his army to Cardueil, where he was defeated and captured by the combined prowess of Gawaine, Lancelot, and Arthur. After he was healed, Brian took Kay’s place as Arthur’s seneschal.
He professed loyalty to Arthur, although in truth he had little love for the king, and harbored a burning hatred of Lancelot – fueled in part by a secret alliance with Claudas, Lancelot’s mortal foe. He tricked Arthur into throwing Lancelot into prison. When Arthur relented and freed Lancelot, Brian defected and joined Claudas. The two later invaded Scotland. The epilogue to Perlesvaus suggests that he killed, or was killed by, Lancelot.
There are, however, many minor knights named Brian or Briant in Malory. I suspect that Brian of the Isles may be identified with the Vulgate’s Sir Brandus des Illes, the former lord of La Dolorous Garde (Joyous Garde) and Doloreuse Chartre.
Source
Perlesvaus | Early 13th century