Japanese literature Ogre or snake in box (tentative)

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Japanese Literature
Three incidents that were sent to the social media project "Tell me about the episode of Oni that happened to you". Contributors or Associates. It was a man who appeared in front of Sanmyo. Mt. Hiyama calls it a "mystery game" and reveals the hidden truth. I want to see the expression on his face when it became known that he had kept it secret, when the thing he was looking for broke, when his position that he thought was superior was in jeopardy - that was Mt. Hiyama's real purpose.