The Great Devil War, book 1: The Devil's Apprentice
Kenneth Bögh Andersen2005

Philip is a good boy, a really good boy, who accidentally gets sent to Hell to become the Devil’s heir. The Devil, Lucifer, is dying and desperately in need of a successor, but there’s been a mistake and Philip is the wrong boy. Philip is terrible at being bad, but Lucifer has no other choice than to begin the difficult task of training him in the ways of evil.
Philip finds both friends and enemies in this odd, gloomy underworld—but who can he trust, when he discovers an evil-minded plot against the dark throne?

Titre original : Den Store Djaevlekrig, book 1: Djaevelens laerling (2005)