Eye of Balor - Wrath of the Old Gods (Young Adult), #3

The Fomorian war reaches a stunning climax in the sixth overall book in the Wrath of the Old Gods series. Steve Symonds is back. And this time, the odds against him have never been greater. What was to be a routine rescue mission turns into disaster when Steve and his allies uncover an alliance of mythical proportions: one that is about to be unleashed upon their supposed safe haven in Valhalla. Betrayed and exiled to the dreaded Aztec underworld of Mictlan, Steve must use his wits and tenacity to escape the latest trap that his enemies have sprung upon him.
All that stands between ultimate victory by the demonic Fomorians is a 13-year old boy- can he succeed when even the gods have failed?