The Letting Spell by Erica Schaef
An invisible evil lurks within the city of Blackpool, victimizing its citizens under the cover of darkness. Bodies are being found in the streets, bodies which have been inexplicably drained of all their blood. Witch Hunter Amara Gray is determined to root out the cause of the vicious attacks, recruiting the help of some of the most notorious members of her trade. When she finds herself faced with an especially powerful, ancient coven, however, Amara must decide whether the danger she is putting herself, along with those she cares for the most in the world, in, is worth the reward.