Raigō (or Tesso) 鉄鼠

Tesso (鉄鼠), the “Iron Rat,” is the embodiment of spiritual fury—the vengeful spirit of a betrayed high priest who transformed into a monstrous rat with iron teeth to devour the sacred scriptures of his rivals.

Meaning and Origin

The name Tesso literally means “Iron Rat” (tetsu - iron, so - rat).

The origin of this yokai is a historical tragedy from the Heian period. It is the Onryō (vengeful spirit) of Raigō, a high-ranking abbot of the Mii-dera temple. After Raigō successfully prayed for the birth of an Imperial heir, the Emperor broke his promise to fund a new ordination platform for Mii-dera due to pressure from a rival temple, Enryaku-ji. Betrayed and furious, Raigō died of a 100-day hunger strike, and his resentment transformed him into this monstrous creature.

Characteristics

Tesso is a terrifying, human-sized rat with a body as hard as stone and fur that bristles like needles. His defining features are his claws and teeth, which are made of solid iron.

He is often depicted wearing the tattered silk robes of a monk, a dark mockery of his former life. Unlike common rats that scurry in the shadows, Tesso moves with a deliberate, spiritual malice, surrounded by a swarm of thousands of common rats that follow his every command.

Legends

The most famous legend of Tesso is the Assault on Enryaku-ji. After his death, Raigō’s spirit led an army of 84,000 iron-toothed rats up Mount Hiei to the rival temple that had blocked his life’s work. The rats didn’t eat food; they “ate” knowledge. They devoured thousands of priceless Buddhist scrolls, wooden statues, and silk paintings, literally eating the history and the spiritual power of the temple.

The rampage only stopped when Mii-dera built a specialized shrine to appease Raigō’s spirit. Interestingly, this shrine (the Hiyoshi Taisha) faces north, directly toward the rival temple on Mount Hiei, as if the “Iron Rat” is still standing guard, making sure his rivals never forget the cost of a broken promise.