The Wicked Day (Arthurian Saga, #4)2003

Born of an incestuous relationship between King Arthur and his half sister, the evil sorceress Morgause, the bastard Mordred is reared in secrecy. Called to Camelot by events he cannot deny, Mordred becomes Arthur's most trusted counselor -- a fateful act that leads to the "wicked day of destiny" when father and son must face each other in battle.