Our Immortal Hearts by Lara Buckheit — book slip

Rory Alarie is destined to be Adelphi's Sacrifice, a role that demands her life for the sake of her kingdom. Constantly monitored by guards and a controlling family, Rory harbors a secret desire to live, not die for the mortal kings or the eradication of supernatural creatures. Enter Quill Godrickson, a famed assassin with a reputation for sparing innocents. Forced by a vampire supreme to eliminate Rory, Quill finds himself drawn to the very woman he's meant to kill. As Rory and Quill navigate a world of duty, desire, and death, Rory faces a choice: die as the Sacrifice or embrace a life with Quill, even if it means becoming one of the creatures she was taught to despise.

First in the Tales from the Wyldelands series.