In Japanese folklore there is a belief that when animals and birds reach advanced age they can transform into yokai. Aosagibi, which translates as blue heron fire, is a phenomenon that occurs when an elderly night heron develops supernatural qualities. Over time the heron's feathers begin to fuse together to form shiny scales that give off a glowing blue light in the dark.

'Night Heron' - Ohara Koson, c. 1910

This yokai also breathes out a golden powder that forms a heat-less fiery light that is carried away by the wind. The glowing blue of an aosagibi is a wondrous sight to behold but like regular herons, this yokai is very timid and will try to avoid human contact. If you are lucky enough to see one, be assured that they are harmless

pandorasboxisopen via Tumblr

However, if you see a glowing light at night, resist the urge to follow it because it might actually be onibi, which can be much more dangerous. Onibi are ghost lights formed by the spirits of dead humans or animals. Although they look beautiful, they have been known to lure people and suck out their life force until they die. Hidama are one type of onibi that can manifest in the shape of birds.

Toriyama Sekien

Aosagibi live near rivers and bodies of water, generally preferring locations where they can hide among thick reeds. This phenomenon was first recorded by Toriyama Sekien in his Konjaku Gazu Zoku Hyakki in 1779 which depicted a night heron with a mysteriously illuminated body. Aosagibi are mostly transformed night heron but other wild birds like ducks and pheasants may also become this yokai.

Onibi taking the shape of birds, Wakan Sansai Zue