An onibi (鬼火) is a kind of kaika in Japanese mythology. According to folklore, they are spirits born from human and animal corpses, which pass as people who have become bitter and turned into fire and appear occasionally.
Sometimes the expressions will-o'-the-wisp or lantern-pumpkin are translated into Japanese to resituate the meaning of the term onibi.
According to the Wakan Sansai Zue composed in the Edo period, an onibi is a blue light like a pine torch and many of them used to gather and humans who come near them have their spirit sucked out.
Moreover, from the illustration of the same Zue, we can guess that it is about two or three centimeters in diameter for a height of about twenty or thirty centimeters and that it floats in the air at about one or two meters from the ground.
According to Yasumori Negishi, in the 10th chapter Onibi no Koto of his essay Mimibukuro of the Edo period, there is an anecdote about an onibi that appeared above Mount Hakone which split in two, flew around, reformed and split again several times.
Nowadays, several theories have been put forward about their appearance and characteristics.
They are generally blue as mentioned before but some are bluish, white, red and yellow. As for their size, some are as small as a candle flame, others are about as big as a human being and some are even several meters long.
Sometimes only one or two of them appear, at other times there are twenty to thirty simultaneously and even at times countless onibi burn and disappear all night.
They usually appear from spring to summer and often on rainy days.
They usually appear in aquatic areas such as wetlands, as well as in forests, meadows and cemeteries and often in places surrounded by natural elements, but rarely in cities.
Some, when touched, are not hot like a fire, but some others burn objects with a heat similar to a real fire.
Since onibi are considered a type of kaika, there are such as those mentioned below and in addition to these, there are also shiranui, chōchinbi, janjanbi, and tenka among others.
One theory holds that kitsunebi is also a kind of onibi but another opinion maintains that strictly speaking they are different from onibi.
Asobibi (遊火, lit. "fire game") is an onibi that appears below the castle and above the sea in Kōchi in Kōchi Prefecture and on Mount Mitani.
It seems very close but moves very far away and when we think of it splitting repeatedly, we must also think of it reconstituting itself as a single being. It is said to cause no particular harm to humans.
In Watarai District of Mie Prefecture, the onibi are called igebo.
Inka (陰火, lit. "shadow fire") is an onibi that appears simultaneously with a bōrei or yōkai.
Kazedama (風玉, lit. "wind ball") is an onibi particular to Ibigawa in the Ibi district of Gifu prefecture. In storms, it appears as a spherical fireball. It is said to be as big as a personal tray and it gives off a bright light.
During the typhoon of the Meiji 30 era (1897), this kazedama appeared from the mountain and floated in the air several times.
The Sarakazoe (皿数え, lit. "counting plate") is an onibi that appears in the Konjaku Gazu Zoku Hyakki by Toriyama Sekien. In the Banchō Sarayashiki known for its ghost stories, Okiku's spirit appears as an inka ("shadow fire") out of the well and is depicted as counting plates.
The Sōgenbi (叢原火 or 宗源火, lit. "fire of the source of religion") is an onibi in Kyoto in the Gazu Hyakki Yagyō of Toriyama sekien.
It is stated that it is a monk who once stole the jizōdō from the Mibu-dera, received the Buddhist punishment and became an onibi and the anguished face of the priest is said to float inside the fire.
This name also appears in the Shinotogibōko, a collection of ghost stories from the Edo period.
The Hidama (火魂, lit. "spirit on fire") is an Onibi from Okinawa Prefecture. It usually lives in the kitchen behind the charcoal extinguisher, but it passes for assuming a bird-like form and flies around and sets things on fire.
Wataribisyaku (渡柄杓, lit. "transverse ladle") is an Onibi from Chii village, Kitakuwada district of Kyoto Prefecture (later Miyama, then Nantan).
It is a bluish-white fireball that floats lightly in the air of mountain villages. It is said to have the appearance of a hishaku (ladle), but this is not what it really looks like but rather seems to carry a long thin tail which has been compared to a ladle by metaphor.
The kitsunebi is a mysterious fire that is the source of many legends, one of which claims that the bone the fox holds in its mouth glows.
Kimimori Sarashina of Michi explains this phenomenon as a refraction of light that occurs near riverbeds Kitsunebi are sometimes considered a type of onibi.
First of all, given the way the details of eyewitness accounts of onibi do not match each other, "onibi" can be considered a collective term for several types of mysterious light phenomena.
Since they frequently appear on rainy days, even though "bi" (fire) is in the name, they are supposed to be different from simply burning flames and to be a different type of luminescent body.
It is particularly interesting to note that in the past, these phenomena were not considered strange.
In pre-C.E. China, it was said that "from the blood of humans and animals comes phosphorus and fire oni (onibi)." The character 燐 at that time in China could also mean the luminescence of fireflies, triboelectricity and not a word refers to the chemical element "phosphorus ".
At the same time in Japan, according to the explanation given in the Wakan Sansai Zue, for humans, horses and cattle who died in battle and whose blood had soaked the ground, onibi are what their spirits turn into after several months and years.
A century after the Wakan Sansai Zue in the nineteenth century and then in Japan, the first to mention them, they are mentioned in Shūkichi Arai's Fushigi Benmō, which states, "The phosphorus of the corpses of those buried turns into onibi."
This interpretation was supported until the 1920s, and dictionaries reproduced it during the Shōwa era and beyond.
Sankyō Kanda, a biologist of luminescent animals, discovered phosphorus in 1696 and since he knew that human bodies were also provided with it, in Japan, the character 燐 was applied and thus it can be assumed that it was mixed with an allusion to China on the relationship between onibi and phosphorus.
In other words, it could be assumed that when the corpses decompose, the phosphorus in the phosphoric acid would give light. In this way, most of the onibi would be explained, but there are also many testimonies that do not correspond to the theory of illumination by phosphorus.
After that, one theory is that it is not the phosphorus itself but rather the spontaneous combustion of phosphine and another theory is that it is the burning methane produced from the decomposition of the corpse that produces the light.
According to a third theory, hydrogen sulfide is produced from decomposition and becomes the source of onibi and finally what would be defined in modern science as a type of plasma.
As they often appear on rainy days, scientists explain onibi as Saint Elmo's fires (plasma phenomenon). Physicist Yoshihiko Ōtsuki has also theorized that these mysterious fires are caused by plasma.
It has also been pointed out that, for lights that appear distant in the midst of darkness, that if they are able to move by suggestion, then there is a possibility that they may simply be related to optical illusion phenomena.
Each of these theories has its own advantages and disadvantages, and since the legends about the onibi themselves are of various types, it seems impossible to conclusively explain the totality of the onibi with a single theory.
Furthermore, they are often confused with hitodama and kitsunebi, and since there are many different theories to explain them and the true nature of these onibi is unknown, there is no real clear distinction between them.