Twilight Guardians. Wings In the Night, #16

Genre-defining vampire romance!They tell 20-year-old Charlie O'Malley that they have a cure for the condition that will eventually kill her-an untested, experimental cure. All they want in return is for her to join their elite team and train to kill the Undead. She takes the deal because it will make her stronger than ordinary humans, strong enough to kill the vampire who took her blood, used her body, and murdered her mother--the vampire who'd lied when he said he loved her. Killion is sure he's the last of his kind, destined to walk in endless solitude, endless night.
Charlie is a spark of light in his eternal darkness. His desire for her is powerful and impossible to resist, and he'll destroy anything that stands between them. Even though it will probably get him killed. Ancient vampire elders Rhiannon and Roland come out of exile, risking their lives to help a furious, raging girl fulfill her destiny as a savior of their kind. But before the tale ends, their lives will be forever changed. Join Maggie Shayne in her most beloved series of all time: WINGS IN THE NIGHT