The Nightmare Girl - Jonathan Janz

Playing with fire has never been more dangerous.
When family man Joe Crawford confronts a young mother abusing her toddler, he has no idea of the chain reaction he's setting in motion. How could he suspect the young mother is part of an ancient fire cult, a sinister group of killers that will destroy anyone who threatens one of its members? When the little boy is placed in a foster home, the fanatics begin their mission of terror.
Soon the cult leaders will summon their deadliest hunters—and a ferocious supernatural evil—to make Joe pay for what he's done. They want Joe's blood and the blood of his family. And they want their child back.