Let Sleeping Demons Lie by Amy Sumida

Deep within the Devil’s house there’s a room where demons sleep. They send their souls into humans and entrust their bodies to Satan. No one has ever disturbed them. No one would dare. Until now.
Demons are dying; their souls taken before they can return to Hell. With only weeks to spare, Vervain and the God Squad attempt to track down the murderer. The trail is twisted and full of traps; blurring the truth in the most devious ways. But this about family, and the Godhunter refuses to turn back, no matter how rough the going gets… or how much she may lose along the way.