Sword of Shadows

Arthur and Kai have escaped the threat of Emrys, but now they must face life on the surface world—and all the fearsome creatures that dwell there. But just as they assemble the beginnings of a fighting force, they discover a vital component to their safety has been compromised. This means a return to Avalon, where Arthur has an unexpected encounter with the untrustworthy Merlin. The magician's orders are clear: Arthur must find the sword to save the surface-dwellers and Avalon. There is no alternative.
Kiri Ung, leader of the Nestines and ultimate controller of the Terran slaves on the surface, needs Arthur in order to ensure of the continuance of his species. With the Nestine Queen dying, failure means ultimate extinction. Wherever Arthur goes, so goes Kiri Ung.
Whoever finds the sword first gains control over all humanity. But simply gaining possession of this powerful artifact is not enough to wield its power. Let the battle commence.