Brian of the Gastine, Brien de la Gastine
A malevolent knight.
He elicited a rash promise from Arthur, which turned out to be the unconditional services of Gawaine. Brien invaded the Lake of the Twins, and forced Gawaine to slay Lord Bleheri, who ruled the land. With Bleheri dead, Brien took over all of his former lands and castles, and imprisoned their knights and ladies.
Meriadeuc, Bleheri’s son, learned of the injustice from his mother and killed Brien to avenge his father’s death. Brien’s son, Galien, continued to plague Meriadeuc’s family but was slain by Gawaine.
Source
Meriadeuc or Le Chevalier aux Deux Épées | c. 1225-1250