Eater of Lives by Jordan Hawk

When a supernatural killer is implicated in a string of cannibalistic murders, federal exorcist John Starkweather must track the demon down with the help of his live-in lover, Caleb Jansen. Caleb is possessed by Gray, a vampire spirit who is a natural demon hunter.

Caleb’s desire to find the killer is tempered by the knowledge his own time is running out. If John can’t successfully figure out how to exorcise Gray before forty days are up, the possession becomes permanent.

When John’s ex shows up, looking for a place to stay while visiting the city on business, tensions mount even higher. Lines are drawn and Caleb learns some decisions, once made, can never be changed.