Dark Harmony by Laura Thalassa

From bestselling indie author Laura Thalassa comes the newly revised and edited final book in her smash-hit dark fantasy romance between a siren and the "bargainer" she owes countless favors to.
There are worse things than death. Things that lurk in the shadows and slip into dreams. Things that once slept...but have now awakened.
For Callypso Lillis, the fae magic that now runs through her veins is equal parts curse and good fortune. While it bonds her to Desmond Flynn, her soulmate and the King of Night, it also leaves her newly vulnerable to the Thief of Souls, who wants to break the Otherworld...and Callie along with it.
But it's not just the Thief whose shadow looms over the fae realms. Des's father, the previous King of Night, has returned from the dead, and he wants revenge on the son who sent him to the grave in the first place.
Time is running out to track down both enemies and stop their plans from unfolding. Callie and Des race...