Tesso (iron rat, a huge rat as large as a bull with a stone body and iron teeth) is a specter of the ancient Japan which was connected to a deep-seated grudge of a priest and a rat. It is also called Raigo nezumi (the priest Raigo transformed by wicked thoughts into a rat) named after the priest who was the origin of the specter.
It is mentioned as tale of a vengeful spirit (vindictive ghost) in setsuwa (anecdotes) dating back to the end of the Heian period and its name appears in "Heike Monogatari" (The Tale of the Heike) and so on.
Behind the background of the tale of Tesso was intense power struggle in the Buddhist region at the end of the Heian period and also it is a historical material to pass on to today the picture of the sogai (rat damage) at that time.
After the death of a Heian period's high priest, Raigo (the Great Priest of Mii-dera Temple) who was famous for being efficacious, he was transformed by wicked thoughts into a vengeful spirit of a rat.
Because Emperor Shirakawa promised that, if efficacious, he would be given as rewards whatever he wanted, Raigo who was an ajari (a master of esoteric Buddhism) of Onjo-ji Temple (Mii-dera Temple) continued to pray for the birth of a prince and accomplished it honorably on December 16, 1074. In spite of this, the erection of Buddhist ordination platform in Onjo-ji Temple which he requested for was not granted after all because of the objection from the competing power, Enryaku-ji Temple on Mt. Hiei.
Building animosity on this matter, Raigo started fasting to pray for dropping into the path of evil Imperial Prince Atsufumi who had been born as a result of his prayer. After 100 days, Raigo changed to a goblinesque figure and died, and around that time a shady, white-haired old monk began to appear at the head of Imperial Prince Atsufumi's bed. Emperor Shirakawa was afraid of Raigo's curse and relied on prayer but without success, and it is said that Imperial Prince Atsufumi passed away at the age of only four. The Raigo's curse did not end with the killing of the prince, but he was transfigured to a giant rat like a bull which had a stone body and iron fangs. Then in order to work off the grudge, he led a large army of 84,000 rats and climbed up Mt. Hiei and bit off statues of Buddha and Buddhist scriptures in Enryaku-ji Temple.
Enryaku-ji Temple feared the curse of Raigo and built a shrine at Higashi-Sakamoto to quell it. Presently the shrine is called Nezumi no Hokura (the shrine built for settling the priest Raigo's grudge).
Juhachimyojin enshrined beside the stone steps at Onjo-ji Temple is called 'Nezumi no Miya' (the shrine sacred to rat goblin) and it is said that it is settling the spirits of rats and built in the direction of Mt. Hiei.
Also it is said that the Tesso was got rid of by a big cat which appeared by the power of Buddhism of a high priest at Mt. Hiei; and, against 'Nezumi no Miya' at Onjo-ji Temple, 'Neko no Miya' (the shrine sacred to cat goblin) was built at Sakamoto, Sakamoto-go, Shiga-gun, Omi Province (presently Sakamoto, Otsu City, Shiga Prefecture) known as the Monzen-machi (a temple town) of Enryaku-ji Temple, as if it were glaring down Enjo-ji Temple, which has been made known today.
In addition, according to another theory the damage caused by the Tesso and rats was not limited to Kinai region (the five capital provinces surrounding the ancient capitals of Nara and Kyoto), but spread to Togoku (eastern provinces); and, it has been told that, when they were about to invade Shimotsuke Province (presently Tochigi Prefecture), Jizo Bosatsu (Jizo Bodhisattva) Enmei (prolongation of life), Kaiun (better luck) and Shori (victory) appeared to keep them off and to put them into a cave at a tumulus. It is said that Atagozuka Tumulus still remaining at Doto, Oyama City is that particular cave, which is the origin of imyo (another name) Raisozuka (a tumulus containing rats).
Also, the historical fact is that Raigo died in1084 and Prince Atsufumi had died in 1077, and so experts are pointing out that Raigo's shiryo (the spirit of a dead person) killing Prince Atsufumi contradicts the historical fact. However, because it is a fact that Raigo made great efforts for the construction of Buddhist ordination platform of Mii-deraTemple, which is the Grand Head Temple of the Tendaijimon Buddhist sect, it is said that the Tesso legend was born in the context of the confrontation between the Tendaijimon Buddhist sects and that it occurred at the end of the tenth century.