What Niby Commands
Speak with Impsy in Felwood.
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.
My imp, Impsy, shall set you on your way.
Oh boy, here we go again.
The entirety of this quest happens at Felwood