Stephen R. Lawhead : Pendragon - Book Four of the Pendragon Cycle

At the dawn of his reign, a young king must prove his greatness . . . or forfeit a realm Arthur is King—but darkest evil has descended upon Britain's shores in many guises. In this black time of plague and pestilence, Arthur's most trusted counselor Myrddin—the warrior, bard, and kingmaker whom legend will name Merlin—is himself to be tested on a mystical journey through his own extraordinary past. So the noble king must stand alone against a great and terrible adversary. For only thus can he truly win immortality—and the name he will treasure above all others: Pendragon.