This quest was marked obsolete by Blizzard and cannot be obtained or completed.
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.