Oath of Six (The Heart of Darkness #1) by Shawn L Johnson
Approx. 101,530 words.
Published on June 19, 2011.
David Tyler receives a desperate summons from his estranged mentor. The old wizard is wasting away, and fears he has been cursed by a former apprentice. The trail leads David and the remaining apprentices into the mythical underworld. There they face exiled shape-shifters and an insane goddess, in order to confront a childhood friend who has become both a traitor and a murderer.