God Save The Vampire

Arabella is a hunter. Everything from vamps to rogue fairies and scorned warlocks find it difficult to fight her ancient mage heritage. But the world starts to change when her coven gets attacked by an invisible force and no one knows what kind of evil powers are involved.

Then on one such mission Ara runs into Caerleon and instead of killing him like she normally would, Ara feels like he doesn't deserve it. She's just about to leave when Caerleon announces he knows what attacked her coven and he is the only para being in the human realm with the power to defeat what's hunting the coven and other paras.

The only catch - Ara has to be his permanent blood donor.