The Wolves of London (Obsidian Heart book 1‪)‬

Alex Locke is a reformed ex-convict, forced back into London's criminal underworld for one more job. He agrees to steal a priceless artefact - a human heart carved from blackest obsidian - from the home of a decrepit old man. But when the burglary goes horribly wrong, Alex is plunged into the nightmarish world of the Wolves of London, a band of unearthly assassins who will stop at nothing to reclaim the heart. As he races to unlock the secrets of the mysterious object, Alex must learn to wield its dark power - or be destroyed by it.