Immortal Hunter by Kait Ballenger

Hunters of the supernatural, the Execution Underground are an elite group tasked with protecting humanity...but at what price?

As an exorcist, David Aronowitz grew up the target of demonic assassins. Now he's a member of the Execution Underground, and hellspawn everywhere fear his name. But when a demon slips into the seductive body of the only woman he's ever loved, David must confront the heartbreak of their past to save her.

The piece of her heart Allsún O'Hare gave to David so long ago left her trapped between two worlds: the Fae and the human. And when David comes to her rescue, fate reunites her with her greatest temptation--and her biggest mistake.

Now, as they're swept together into a wicked game with the demon who controls her, David must decide if saving Allsún's life is worth sacrificing his own--and the future of humanity itself.