Search Shatter Scar Vale in Felwood for the remains of Kroshius. When you locate the remains, use the Fel Fire near them and await Kroshius' return. When the infernal has awoken, slay him and take from his corpse Kroshius' Infernal Core.
Return Kroshius' Infernal Core to Niby the Almighty in Felwood.
Kroshius lays in a heap to the northeast, in the Shatter Scar Vale. Take this Fel fire and use it on his remains. The fire should reanimate the fallen infernal.
At this point, I fully expect him to kill you and any friends that you may have brought along - and trust me, you will want to bring along some friends.
If by some miracle you happen to kill Kroshius, take his infernal core and return it to Niby.
