On the way to classroom, you receive a suspicious letter.
The sender claimed to be a demon, who wants you to offer yourself as a sacrifice.
Originally they will force you to do so, but they say they will give you a chance by playing their little game.
They altered everyone's memory, and will appear as one of your classmate.
When you go home, you must choose one among three candidates to accompany you. If you choose wrongly, you will lose the demon's game.