Enemy of God by Bernard Cornwell

Arthur has won his bloody victory at Lugg Vale and the kingdoms are finally united. Mordred’s throne is safe, Guinevere is to bear Arthur a child, and Lancelot is to marry Ceinwyn. After one last battle against the Saxons, Arthur will rule a peaceful, orderly land. But, unlike Merlin, Arthur has forgotten the Gods, who thrive on chaos. Merlin, weaver of charms, knows that if the Gods are to be restored he must bring together Britain’s thirteen sacred objects. Derfel, the stalwart of Arthur’s shield wall, is drawn into Merlin’s intrigues and Arthur’s plans are thrown into turmoil…