The Arthurian Saga 04 - The Wicked Day

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.

The Winter King by Bernard Cornwell

Essentially this is a modern political thriller, told in flat American diction. Narrated by Derfel, an ordinary, likable man who rises through the ranks to become Arthur's friend and advisor in peace and war, the story doesn't follow the traditional patterns. Mordred is Uther's infant grandson, the legitimate king; Arthur is one of Mordred's guardians, sworn to hold the kingdom against the Saxon warlords until Mordred comes of age. Warfare is incessant. Arthur's dream of peace and unity seems unattainable.