Tesso

thumb|upright=0.8|"Tesso" (鉄鼠) from the Gazu Hyakki Yagyō by [[Toriyama Sekien. The text on the top left reads, "it is the place known to the world where Raigō's spirit turned into a rat" (頼豪の灵（霊）鼠と化(かす)と世尓(に)志(し)る所也)]] thumb|upright=0.8|"Mii-dera Raigō Ajari Akunen Nezumi to Henzuru Zu" from the Shinkei Sanjūrokkai Sen by [[Tsukioka Yoshitoshi. It faithfully depicts the writings in the Genpei Jōsuiki.。]] thumb|upright=1.25|"Raigō Ajari Kaisoden" (頼豪阿闍梨恠鼠伝) by Takizawa Bakin and illustrated by [[Katsushika Hokusai. The big rat that appears in front of Nekoma Michizane is written in the text to be the size of about an ox, but the illustration exaggerates it and depicts it as a giant beast.。]] 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.

==Story== ===Heike Monogatari=== 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.