Kemurikage

The Kemurikage are spirits of Fire Nation legend. They are feared by the children of the capital of the Fire Nation as they supposedly abduct them for misbehavior.

They are revealed to be true when they appear before Ukano and berate him for his failures as the New Ozai governor and tells him to overthrow Zuko and reinstate Ozai to the throne or he will lose everything dear to him.