Oaths of Blood. The Ascension Series, #2

The hordes of Hell are marching, and it's going to take a demon to stop them. Werewolves are immune to every illness and can heal any wound. It should be impossible for one to become possessed by a demon. But that's exactly what Seth Wilder is facing: a werewolf gone insane from possession. He has no choice but to deliver her to the only exorcist in America, Elise Kavanagh, who also happens to be a powerful demon known as the Godslayer.
Elise is in hiding when Seth and Rylie Gresham, Alpha werewolf, arrive seeking her help. She agrees, but everything has its price. What they learn about the possessed werewolf changes everything--Hell and Earth, the pack, and the future of the entire world...