If I am to teach you the secrets of summoning an infernal servant, you must destroy Kroshius and bring me the infernal core from his burning remains. This, however, is not as easy as it sounds. Kroshius has all but burned out completely. You must reignite his corpse and raise him from the rubble. To do this, however, you must gather some components from other demonic entities.$B$BMy imp, Impsy, shall set you on your way.

Speak with Impsy in Felwood.