A Hunter's Promise — Gwendolyn Cease

Aislinn Flaherty makes her home at a mental hospital. She's not mentally ill, but after a traumatic childhood event, the hospital is where her family puts her. Permanently. But when scary paranormal things come out to stalk the residents, she starts hunting.

Charon is an Ancient, a vampire who must drink human blood. His role as a Hunter makes him, and his fellow Hunters, the equivalent of the paranormal Special Forces. He spends his days sleeping and his nights hunting the things that lurk in the darkness.

When Aislinn ends up in Charon's world, it's not something either of them wants. But sometimes, what you think you want and what you need are two totally different things.