The son of King Locrinus and Queen Gwendolen of Britain, and thus grandson of Corineus.
His right to the accession was upheld by his mother, who, after Locrinus had abandonded her for his mistress Estrildis, mustered an army in Cornwall and, in a battle near the River Stour, killed her wayward husband. Gwendolen then reigned for fifteen years before abdicating in favor of her son.
He ruled for forty years and his sons, Mempricius and Malim, contended each other for the throne after his death.
Source
Historia Regum Britanniae | Geoffrey of Monmouth, c. 1138