The Last Enchantment - Mary Stewart

Crowned and recognized as High King, Arthur melds the country together in a time of promise, as Merlin works in the shadows. But while unchallenged on the battlefield, sinister powers plan to destroy him in his own Camelot. 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.