Wizardborn. Book 3 of the Runelords

Reeling from the battle at Carris, the young king Gaborn Val Orden finds that he has lost the powers that let him protect his people. As he struggles to rid his land of reavers, he discovers that the creatures are more resourceful and terrifying than even he imagined. Desperate for any weapon that will aid him in this battle, all of his hopes come to rest upon the nine-year-old girl, Averan, the Wizardborn apprentice to the earth wizard Binnesman.
Despite her fears, he must convince her to lead him in battle against the reaver lord of the underworld. Meanwhile the ailing Wolf Lord, Raj Ahten, learns that a reaver horde is decimating his own homeland. There, he confronts his enemy, and finds that he must choose to strip himself of all humanity or face ultimate defeat .