Darkyn's Mate (#3, Rhyn Eternal) - Rhyn Eternal, #3

Caught between Fate and the Dark One ... Stuck in Hell, human-Deidre struggles to hold onto her hope that she can change the course of events that landed her as the mate of Darkyn, the Dark One. The chances of her leaving Hell plummet when the Dark One takes a drastic step to ensure he never loses his latest possession and blood binds her in an ancient rite from the time-before-time. In doing so, he is forced to share his power with her.
Rather than despair, she begins to see that she has unexpectedly found her place in the world: protecting innocent souls from the bloodlust of the demon she's forever bound to. Rather than accept her place at his side docilely, she begins to use her newfound influence to help those she cares about, including Gabriel, the former lover she's not yet ready to let go of, even knowing she'll never leave Hell.