Tesso (鉄鼠) is a Japanese yōkai related to the vengeful spirit (onryō) of the Heian period monk Raigō and a mouse. The name “tesso” is a name given by Toriyama Sekien in the Edo period collection of yōkai pictures, the Gazu Hyakki Yagyō, and this yōkai can also be called the Raigo-derived name Raigō-nezumi (頼豪鼠) as from the Enkyōhon (延慶本, alternatively read Enkeihon), a yomihon version of the Heike Monogatari, or Mii-dera-nezumi (三井寺鼠) derived from Mii-dera in Ōtsu, Shiga Prefecture as in the Edo period yōkai themed kyōka picture book, the Kyōka Hyaku Monogatari. They became known starting in the Heisei period from its adoption in the mystery novel Tesso no Ori (The Tesso’s Cage) by Natsuhiko Kyogoku.
According to the Heike Monogatari, in the Heian period, Raigō, on the basis of a promise with Emperor Shirakawa of receiving as much reward as desired if it had effect, continuously prayed for the birth of the emperor’s crown prince, and in 1074 (in the Jōhō years) on December 16, it was finally succeeded. As a reward, Raigō requested for the construction of an ordination platform (kaidan) building for Mii-dera, but opposition forces from Enryaku-ji at Mount Hiei obstructed this and prevented it from being granted.
In resentment, Raigō then proceeded to make prayers to drag the crown prince who was born from his prayers, Prince Atsufumi, down to the clutches of evil, and went into a fast. After 100 days, Raigō ended up looking like an evil ogre and died, and starting since then, an ominous white-haired old priest would appear by Prince Atsufumi’s pillowside. Emperor Shirakawa was fearful of Raigō’s curse and grasped around for prayers, but they had no effect, and Prince Atsufumi died barely at the age of 4.
In the yomihon versions of the Heike Monogatari such as Enkyōhon (延慶本, alternatively read Enkeihon) and Nagatohon (長門本) and in the different book Genpei Jōsuiki, among other sources, the grudges of Raigō became a giant rat and ate away at the sacred books in Enryaku-ji. Enryaku-ji was fearful of Raigō’s grudge and built a shrine at Higashisakamoto to enshrine Raigō as a god in order to quell this grudge. The name of the shrine has been passed down to be “Nezumi no Hokura” (鼠の秀倉, The Rat Shrine). It is also said that after this, large rats have been called “Raigo-nezumi.”
According to the military chronicle (Gunki monogatari) Taiheiki, the grudges of Raigō turned into 84,000 rats with stone bodies and metal teeth and climbed up Mount Hiei and ate away at not just the sacred texts, but also the Buddha statues.
In the late Edo Period, the yomihon author Takizawa Bakin wrote Raigō Ajari Kaisoden (頼豪阿闍梨恠鼠伝) based on the Raigō legend. While the elegant Shimizu Yoshitaka (水義高, also known as Minamoto no Yoshitaka or 源義高), the orphan of Kiso no Yoshinaka was traveling across several provinces, Raigō appeared to her in a dream and talked about how because Yoshinaka once contributed a request form to Raigō’s mini shrine (hokora) to become Seii Taishōgun, so he will offer his help to Yoshinaka, and told her about how Nekoma Michizane has been targeting her life due to having a grudge against her, and instructed Yoshinaka on some rat sorcerery.
In the story, the scene when Mitsuzane tried to cut down Yoshinaka and a giant rat appeared and stopped this, the scene when Yoshinaka called out an eerie person with the face of a rat in order to lure out her father’s enemy Ishida Tamehisa, and the scene when Yoshinaka stopped Michizane’s torture of her mother-in-law by summoning countless rats, among other scenes, were all given illustrations by the ukiyo-e artist Katsushika Hokusai.