Sayounara, Watashitachi ni Yasashikunakatta, Subete no Hitobito

In seeking revenge against the seven individuals who killed her sister, the girl sacrificed her life.

In the rural town of Agata, surrounded by mountains on all sides, Shiori Nakagawa, a student at the local high school, had become a truant due to bullying. One day, Mei Satou arrived at Shiori's house as a roommate. Mei, who kept her heart closed to everyone, confided in Shiori about her purpose in coming to this town.

"I want to slaughter all seven people who drove my sister to death."

Three years ago, Mei's sister, Akari, had taken her own life after experiencing horrendous bullying in this town. Mei had returned here for the sake of revenge. Mei planned to carry out the bloodstained ritual "Okakashitsutsumi," passed down at the Agata Shrine, and house the giant snake god "Okakashisama" within herself to execute the plan of killing the seven individuals.

On the night of the summer solstice, Mei successfully performed the ritual. From that point onward, one by one, Mei, with the power of the god, began killing the individuals involved in her sister's death, alongside Shiori. Amidst the days of revenge and escape, the two gradually developed romantic feelings for each other. However, Mei had a hidden truth from Shiori: those who performed "Okakashitsutsumi" must ultimately offer their own souls to the god, meaning Mei's death was inevitable.

(Source: Shogakukan, translated)