The Kingmaking - Pendragon's Banner Trilogy

Pendragon arrives at Gwynedd to ally himself with Cunedda against the tyrant Vortigern, but is slain in battle. A boy, Arthur, is revealed as his son, but he must bide his time. Not until the Saxons plunge the country into chaos is he able to claim the kingship and his love Gwenhwyfar as his.