The Last Enchantment by Mary Stewart, Hardcover (First edition)

Arthur Pendragon is King! Unchallenged on the battlefield, he melds the country together in a time of promise. But sinister powers plot to destroy Camelot, and when the witch-queen Morgause -- Arthur's own half sister -- ensnares him in an incestuous liaison, a fatal web of love, betrayal, and bloody vengeance is woven.