Enemy of God (The Arthur Books #2)

Arthur, having defeated the last holdouts of civil war in southern Britain, has secured King Mordred's throne. But the unified kingdom seems no steadier, its balance threatened by Merlin's ceaseless - and some say futile - quest for the last of the Thirteen Treasures of Britain, by the conflict between the ancient religion and the new Christianity, and by Britain's war against the Saxons. Arthur must face other foes as well, foes more powerful and more dangerous - becuase they pose as friends.