The Last Enchantment (Arthurian Saga, #3)

Arthur Pendragon is king! Crowned and recognized as High King, Arthur is unchallenged on the battlefield, and he melds the country together in a time of promise, as Merlin works in the shadows. 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.