Make Him Roar by Ember-Raine Winters

Running from the supernatural taskforce is never advised, but for Rory, the alternative is a fate worse than death.

Raised by the supernatural taskforce, Rory is the best spy they have, but her time as a spy is coming to an end. One last job, and then she’s off to the breeding program.
Stowing away on Abbadon’s boat changes everything. He plans to punish her for sneaking on his ship, but when he realizes she’s exactly who she’s been looking for, her fate takes a turn that’s far worse than what he originally had in mind.
As the lost princess of a dying realm, Abbadon wants her for his own. There’s only one problem. She has to mate with a prince of hell…or the realm will die.