Pendragon (The Pendragon Cycle‪)‬

Arthur is King, but treachery runs rampant throughout the beleaguered Isle of the Mighty. Darkest evil descends upon Britain's shore in many guises. Fragile alliances fray and tear, threatening all the noble liege has won with his wisdom and his blood. His most trusted counselor—the warrior, bard and kingmaker whom legend will name Merlin—is himself to be tested on a mystical journey back through his own extraordinary past. So in a black time of plague and pestilence, it is Arthur who must stand alone against a great and terrible adversary. For only this way can he truly win immortality—and the name to treasure above all others: Pendragon.