An Onibi is a type of Will O' The Wisp from Japanese folklore that appears as small balls of flame and will swarm victims, draining their life-force in the process - for this reason thy are considered the most dangerous of the many different spirit-flame creatures found in the region.
Onibi are seen as spirits of the dead, mischievious yokai or the creations of kitsune and were likely the result of mistaken identity in ancient times of swamp gas, optical illusions and certain species of glow-fly.