Ironcrown Moon — Julian May

Julian May continues her "rollicking sword-and-sorcery saga" (VOYA) with the tale of a king under siege, a knight caught between light and darkness, and a land poised on the edge of ruin...

Through strength and treachery, King Conrig has united the warring island of High Blenholme -- but his enemies are ever-watchful. Conrig's illicit magical abilities are known to them, and his children show the same abhorrent gift. Then word comes that his first wife, thought long dead, has been seen. And with her, a son who may be Conrig's true heir.

More danger arises when the Trove of Darasilo -- a collection of powerful moonstone sigils and ancient texts -- vanishes from its secret crypt. In the wrong hands, it would seal Conrig's doom. And the hands holding it are almost certainly those of Prince Beynor of Moss, who would deal with even the most malevolent magical specter to regain his lost realm.

Conrig's only ally is his childhood friend, Snudge, now a titled knight. But even Snudge finds his loyalty tested. For he knows Conrig's greatest weakness is his lust for power -- and that the Trove could be his key to acquiring it...