Agravaine was the brother of Tristan de Bois and Queen Ygraine, King Uther's wife. His sister died after Uther went to the sorceress Nimueh and made her use magic to conceive a child, Arthur. The price for conceiving a child using magic was her own life. One of the reasons he allied with Morgana was most likely because of her death and the fact that Arthur was the result of her death. His other sibling, a brother called Tristan was also angry at his sister's death and was killed by Uther when he tried to avenge her. Having lost two siblings, Agravaine hated both Uther and Arthur and probably believed that they had no right to rule Camelot. Agravaine sided with Morgana many years later to probably take his revenge on them and to assist her in taking the throne.