Excalibur by Bernard Cronwell

The final novel in the Warlord Chronicles series by bestselling author Bernard CornwellArthur's final test of courage is upon him . . .Arthur has crushed Lancelot's rebellion, but at a cost. Guinevere's betrayal has left him reeling, and his Saxon enemies seek to destroy him while he is weak. Chaos threatens to engulf Britain. Yet Arthur is a military genius and noble leader. As the battle draws close, he prepares to fight his way to victory at Mount Badon and also win back the woman he lost. But in the final journey of the warlord, the intrigues of Mordred, now the adult heir to the throne of Britain, and the magics of the priestess Nimue could prove to be Arthur's downfall.