Tesso, the Iron Rat, is how the abbot Raigo of the Miidera temple, who transforms into a half-man half-rat hybrid, is known in Japanese mythology.
Tesso differs from many yokai in that he is a single character. There is only one Tesso.
The word tesso is formed from the kanji 鉄 (te: iron) and 鼠 (sso: rat).
This yokai was given the name Tesso by Toriyama Sekien in his yokai collection called Gazu Hyakki Yako (画図百鬼夜行: Illustrated Night Parade of the Hundred Demons),
although the character is much older. Until Toriyama Sekien named him Tesso he was known as Raigo Nezumi (頼豪鼠), meaning Raigo the Rat.
The story begins with Emperor Shirakawa, who was desperate for an heir to the throne. He asked for help from the abbot of the Miidera temple, a powerful Buddhist monk named Raigo. Emperor Shirakawa promised Raigo great rewards if he would use his spiritual powers to grant him a son.
Raigo accepted the offer and gave himself to meditation, prayer and magic. Soon Emperor Shirakawa's son, Prince Taruhito, was born. Raigo went to the Emperor to receive the promised reward, and asked only for what was needed to build an ordination platform at his temple in Miidera.
The emperor had no objection to oblige, until political problems between temples interfered.
Miidera had a rival temple, the mighty Enryaku-ji on Mount Hiei in Kyoto. The monks of Enraku-ji were no ordinary, peaceful monks, but a terrible army of militant warriors feared throughout Japan.
It was said that the emperor of Japan could influence all but three things: the blowing of the wind, the rolling of dice in a dice cup, and the monks of Enraku-ji.
Despite both being of the Tendai Buddhist sect, Miidera and Enraku-ji had split into two different factions after the death of their founder. Enraku-ji would not allow new Tendai monks to be ordained at Miidera, a privilege they reserved for themselves.
The emperor had no choice but to break his promise to Raigo. He asked if there was anything else he could give him, but Raigo was adamant. So inflexible, in fact, that he went on a hunger strike and died after 100 days, cursing the emperor with his last breath.
At the moment of his death, it is said that a white figure appeared before the cradle of the four-year-old prince Taruhito, who died soon after. What Raigo had given, Raigo took away.
What happened next was strange: so far it would be the typical ghost story with Raigo returning as a yurei. But the story doesn't end here. Raigo used black magic to ensure that he would be reborn after his death as a terrible yokai.
He transformed his body into the form of a giant rat as big as a human, with a body as hard as rock and with claws and teeth of iron.
The now named Raigo the Rat invaded Enraku-ji with an army of rats, devouring its rare and valuable Buddhist scriptures, even eating the statues of the Buddha himself.
This reign of rat terror continued until a temple was built to placate Raigo, transforming him from a terrifying emissary of vengeance into a protective kami spirit. Because that's how evil spirits did things in Heian period folklore.
Ancient texts describe Raigo as an onryo, the name for the vengeful spirits so popular in Japanese horror filmography. Raigo would not be seen as an onryo today: his transformation into a rat would make him more monster than ghost.
But in the Heian period the word onryo had a more specific meaning, being something that cursed or had a grudge against the emperor or the imperial family. And this definition fits Raigo perfectly.
The story of Raigo comes from the Heike Monogatari (平家物語: The Story of Heike) an epic poem from the Heian period that recounts the wars between the Heike and the Taira that split Japan into two factions fighting for the throne.
The Heike Monogatari is sometimes said to be Japan's version of the Odyssey, freely mixing historical facts with supernatural themes and mythology.
Since the Heike Monogatari comes from traditional oral storytelling, there are several versions of it with variations on the story of Raigo the Rat.
In one of the oldest versions, the Engyo Hon (延慶本: Book of the Engyo Period), the story ends with the death of Prince Taruhito. In later versions Raigo appears increasingly monstrous.
The 48-volume Genpei Seisuiki version features Raigo attacking Enraku-ji with his armada of rats, and in the epic and historical Taiheiki (太平記: Record of the Great Peace) of the 14th century Raigo is described as having a body of stone and teeth and claws of iron.
This Raigo not only ate the sacred texts of Enraku-ji, but also Buddha statues.
Although the story of Raigo the Rat is fictional, most of the main characters existed historically.
Shrine records state that Raigo was the abbot of Miidera, and that at one point he asked Emperor Shirakawa for funding to build an ordination platform, which was denied.
No doubt the rival temple Enraku-ji had something to do with the refusal. At that time, Enraku-ji's power was absolute.
The only person who was not involved in the affair was Prince Taruhito. Records place his death in 1077, while Raigo died in 1084. This contradicts the facts of the legend.
Rats, of course, were a source of fear for the fragile literary collections of all the temples in Japan. So it is not surprising that a dual combination of angry spirit and scroll-eating rat was a natural mix for a Kaidan.
Raigo the Rat was such a popular character that other writers continued the story after the Heike Monogatari.
For example, a collection of Tanka poems from the city of Otsu in Shiga Prefecture called Kyoka Hyakumonogatari (狂歌百物語: One Hundred Stories of Satirical Poems) includes the poem Raigo of Miidera and rewrites the story of the Heike Monogatari.
During the Edo period, author Gyokutei Bakin wrote the story Raigo Ajari Kaisoden (寺門伝記補録: The Story of Abbot Raigo Who Transformed into a Monstrous Rat), illustrated by the famous ukiyo-e artist Katsushika Hokusai.
Gyokutei sets Raigo in a different historical narrative, telling the story of Shimizu Yoshitaka (also known as Minamoto no Yoshitaka), the orphaned son of Minamoto no Yoshihara.
Yoshitaka was on a pilgrimage to holy places when he had a vision of Raigo, who told Yoshitaka that he would teach him the secrets of black magic and help him amass an army to take revenge on those who had murdered his father.
All Yoshitaka had to do was write an official request for help, and drop it off at Raigo's shrine along with a donation.
Yoshitaka of course did as he was told, and soon had Raigo's ability to shapeshift and be able to control the rats.
As an additional twist, Yoshitaka is chased by Nekoma Mitsuzane (whose name ironically begins with the kanji for "cat" as a nod to the traditional game of cat and mouse).
In another scene, Nekoma finds Yoshitaka and is about to kill him when a giant rat steps in to defend Yoshitaka. In a different scene, Nekoma is torturing Yoshitaka's mother-in-law, and Yoshitaka leads an army of rats to her defense, saving her.
Hundreds of years later, Raigo remains in the popular imagination. Modern author Kyogoku Natsuhiko used Raigo's story as the basis in his mystery novel Tesso no Ori (鉄鼠の檻: The Cage of Tesso).
There are several temples supposedly dedicated to Raigo, each claiming to be the temple that ended the scroll-eating vendetta.
In Hyoshi Taisha, in the Sakamoto district of Otsu City, Shiga Prefecture, there is a temple called The Rat Temple that some link to Raigo. The temple records, however, state that it is dedicated to the Rat God of the Chinese zodiac, and not to Raigo.
The Miidera temple has the more obvious connection, and has a small monument and temple dedicated to Raigo, also called the Rat Temple. This temple directly faces Mount Hiei in Kyoto, and is said to have been erected in defiance of Enraku-ji's role in the curse of Raigo.
However, Mount Hiei has its own temple: the Cat Temple, which directly faces Miidera. Some suspect that the two temples are connected by an older legend about a monk who summoned a giant cat to wipe out a giant rat that was threatening that area.
It may be that those two Rat Temples were re-dedicated to fulfill the interest of the story. Just like relics in Catholic churches, a temple or artifact related to a popular legend can mean a lot of attractive tourist dollars, and neither Buddhist nor Shinto temples let facts get in the way of a good story. Especially those that attract tourists.