1983 Mary Stewart novel about the last days of the Arthurian legend (see Arthurian Mythology and The Chronicles of King Arthur for an extensive discussion), and in particular Mordred, King Arthur's bastard son and nephew by incest with his half-sister Morgause.
Mordred the traitor, perjurer, and adulterer is as much an invention as the lover and great knight Sir Lancelot, and the roles played by both in the tales of "King Arthur and his Noble Knights" are filled with the absurdities inevitable in a long-drawn series of stories.
—Mary Stewart in her Author's Note to The Wicked Day
In The Wicked Day, Mary Stewart portrays Mordred as the victim of an unfortunate fate, rather than the villain he is conventionally made out to be. As a result, at the end of the book she not only provides a summary of the myth that inspired, but notes as to what informed her decision to adapt his character. I enjoyed this book less than its predecessors, the author's Merlin Trilogy—The Crystal Cave, The Hollow Hills, and The Last Enchantment. Those three are told from the perspective of the enchanter Merlin (Myrddin), whereas The Wicked Day is told in the third person, and without the wizard's omniscience to explain events that no other character could have witnessed, the narrative loses a lot of coherence. However, I did appreciate her efforts to reconcile the contradictions of the myth---her version provides reasonable motivations for many of the players' actions, some of which were, as her Author's Note pointed out, somewhat inexplicable.
The Wicked Day by Mary Stewart. New York: William Morrow and Company. ISBN 0-688-02507-2