The Wicked Day — Mary Stewart

The Wicked Day is the gripping story of Mordred, bastard son of King Arthur by incest with his half-sister Morgause, witch-queen of Lothian and Orkney. Morgause sent the child to the Orkney Islands to be reared there in secret, in the hope that one day he would become, as Merlin the Enchanter had prophesied, the doom of her hated half-brother.

When Mordred is taken from his rude life as a fisherboy in the islands and suddenly thrust into the full panoply of the High King Arthur's court, he learns of his true parentage and rises to a position of trust in his father's kingdom. But, as the plots and counterplots of the last part of Arthur's reign unfold, Mordred is drawn into the tangled web of tragedy that is the climactic drama of the Arthurian legend.