One of the rebel kings at the beginning of Arthur's reign, he pledged to bring 5,000 mounted men to the battle of Bedegraine. In that battle Brandegoris, King Idres, and King Agwisance unhorsed Sirs Griflet and Lucas.

Surviving the battle, Brandegoris seems not to have joined the second coalition of rebel kings, for he is almost certainly to be identified with King Brandegore, on whose daughter Sir Bors begat a son, Helin le Blank. Bors revisited Brandegoris when Helin was fifteen years old and, with Brandegoris' consent, brought the boy to Arthur's court.

King Brandegoris himself, however, seems never to have officially joined Arthur's court. Brandegoris also fathered a son, Evadeam.