Suiten Nikkou Amaterasu Yano Shizu Ishi

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Eightfold Blessings of Amaterasu on the Weight Stone under the Sunlit Watery Heavens.
The Sweets Spirit Cas-ko’s Noble Phantasm. She doesn’t use it much.
Within EXTRA it is described as a mirror, but it is the Tamamo Shizuishi, a very great sacred treasure.
The Shizuishi was enshrined in Izumo, and it is a sacred treasure said to have been brought to Earth from heaven by Takehideri no Mikoto, and is thought to be a vessel that holds the spirit of Izumo Daijingu.
According to the writings of Japanese scribes, it was taken from Izumo at the request of the Imperial court and was afterward enshrined in Kawachi.
It’s probably the Yata Mirror that came afterwards, in short, it’s the vessel that holds the spirit of Amaterasu Oomikami.
The Shizuishi is thought to be the prototype of Mononobe’s Ten Sacred Treasures (Tokusa no Kandakara), and is said to have the power to activate the soul and life force and even revive the dead.
This is far too powerful and cheat-like, so in the game we made it to just provide unlimited magical power.
This is once again digression, however the Ookuninushi that enshrines the Tamamo Shizuishi is basically the god of gods. Depite Cas-ko appearing as she does, she is indeed a top rank God Spirit class Servant.

Fate/EXTRA Material: Encyclopedia of Fate/EXTRA