The Last Enchantment — Mary Stewart

Arthur is King! But while he is unchallenged on the battlfield, sinister powers plot to destroy him in his own Camelot. When the rose-gold witch Morgause, Arthur's half-sister, ensnares him into an incestuous liaison -- and bears his son, Mordred, to use to her own evil ends -- a fatal web of love, betrayal and vengeance begins....

Seen through the eyes of Arthur's prophetic protector, Merlin -- whose own mysterious powers are in dire danger -- here is a tale rich with the majesty and turbulence of the magnificent legend it tells....resplendent with Mary Stewart's own special magic.