Showing Night Pleasure by Sherrilyn Kenyon

AAmanda Devereaux has a unique family. Her mother and older siblings are witches and psychics, and her twin sister is a vampire hunter. All Amanda wants is a quiet, normal life, but when she finds herself the target of an attack meant for her twin, she wakes to find herself handcuffed to a sexy, blond stranger. And while Amanda's first thought is that this might be another of her sister's attempts at extreme match-making, it soon becomes clear that Kyrian of Thrace is not boyfriend material.

For Kyrian is a Dark-Hunter: an immortal warrior who has traded his soul for one moment of vengeance upon his enemies. He spends his eternal days hunting the vampires and daimons that prey upon mankind, and he is currently on the hunt for a very old and deadly daimon named Desiderius who has deemed it sport to handcuff Kyrian to a human while he hunts him. Now Kyrian and Amanda must find a way to break their bond before they give into their dangerous attraction to one another. Or Desiderius kills them both...