Hellwalkers — Alexander Gordon Smith

The Devil's Engine: Hellwalkers concludes an explosive trilogy about the secret battle for control of an all-powerful ancient machine which can grant you anything you wish for, in exchange for your soul.

In the third and final book of Alexander Gordon Smith's Devil's Engine series, Marlow and Pan are in hell. Literally in hell. Faced with the awful truth of being trapped in the underworld for an eternity―of Pan being trapped―Marlow makes a final deal with the Devil, a deal to go home. But he should have known: there is no real escape from a fight with the ultimate enemy. And when all hell breaks loose, it will be a war to end all wars―with demonic creatures spilling into the streets, monsters emerging from the shadows. Only the Hellraisers stand in their way, and they're not sure this is a battle they can win. They have no powers, they have no weapons. But they have each other, and they have hope, and they know how to kick ass.