The Devil’s Pets by Alex Siegel

The Paranormal Enforcement Administration is a top-secret government agency dedicated to battling vampires, werewolves, witches, and other supernatural creatures. Their missions are fierce and deadly. A man named Stony is the leader of an elite team within the agency, and he has the power to make his skin as hard as stone. He once served a demon, but now he destroys evil instead. In The Devil's Pets, Stony must stop a serial killer who kidnaps and tortures children. This Satanist keeps three slaves to assist in his bloody rituals. Diana is the most talented with the ability to transform into a panther. With some luck, Stony can turn her to the side of good over the course of a gruesome mission.