The Medium - Liminality, #1

It begins with the breaking of a gentle monster. A medium cannot kill. A medium must not kill. And a medium should never become a vampire. Lenny played human, kept his head down, never took a life, until Sebastian came. Torn away from his comfortable life, he is plunged into an endless night of manipulation, death, and blood. The only light in the dark is Kim, a young wizard tasked with destroying Sebastian.
She is determined to save Lenny from the monster controlling him, but the monster growing inside him will be harder to kill. Unknown to all, the existence of a vampire medium has sparked a war.