Dark Harmony (The Bargainer Book 4) by Laura Thalassa

There are worse things than death. Things that lurk in the shadows and slip into your dreams. Things that have no business existing. Things that once slept... but have now awoken.

For Callypso Lillis, the fae magic that now runs through her veins is equal parts curse and good fortune. For the very thing that bonds her to Desmond Flynn, the King of the Night, also makes her vulnerable to the Thief of Souls, a man who wants to break the world... and Callie along with it. 

But it’s not just the Thief whose shadow looms over the Otherworld. Des’s father is back from the dead, and he wants revenge on the son who sent him to the grave in the first place. 

Des and Callie must figure out how to stop both men, and time is running out. Because there are forces at play working to tear the lovers apart once and for all... and unfortunately for them, death is no longer the worst thing to fear.