an evil sorceress (=a woman who has magic powers) who is the half-sister of King Arthur in old stories. ("Morgan the Fairy") Enchantress in Arthurian legend. Skilled in the arts of healing and changing shape, she ruled Avalon, the island where King Arthur retreated to be healed of his wounds after his last battle. She had learned her magic powers from books and from Merlin. In other stories she is Arthur's sister and enemy, and seduces him to produce a son who later kills Arthur