13-nin no Uruwashiki Kedamono

On the day of the school festival, Mamoru and his childhood friend Kanade plus eleven other people consisting of teachers and students are invited into a mysterious event held in the basements of the school church.

After they have arrived an announcement tells everyone to take a seat and states them the rules of the game about to begin. One random person is chosen to be a Devil who hunts villagers at night. Each day villagers must decide on who to execute and the game continues until the real Devil is caught or they manage to kill everyone save for one other person.

Still believing it is nothing but a short special event planned by the school, reluctant Mamoru and twelve other players start with the first Judgement phase. The playful atmosphere turns into one of horror when the players discover a grotesque body of the first judged person and notice that the entrance is locked, their cries for help going unnoticed.

The announcement sounds again; "Breaking the rules means instant death and the only way out is finding the Devil."