Sayonara, Watashitachi ni Yasashikunakatta, Subete no Hitobito

Akadacho is a rural town surrounded by mountains on all sides.

Shiori Nakagawa is suppose to be a student at the high school in town, but he has been skipping school because of bullying.

One day, Mei Sato moves into Shiori's house as a renter.

The normally reticent Mei tells only Shiori the true purpose of her visit to this town.

"I want to kill the seven people who forced my sister to her death."

Three years ago, Mei's older sister Akari was bullied so severely in this town that she took her own life.

Mei has returned here to avenge her death.

She plans to kill the seven bullies by performing a bloody ritual called "Okakashitsumi," a tradition at the Akada Shrine, to summon the spirit of the snake god "Okakashi-Sama" into her body. On the night of the summer solstice, Mei succeeds in the ritual.

One by one, she and Shiori, with the help of the god's powers, kill the people involved in her sister's death.

During their days of revenge and escape, the two develop romantic feelings for each other.

However, Mei is hiding something from Shiori. The person who performs "Okakakatsutsumi" must offer her soul to the god in exchange. In other words, "Mei's death" is inevitable.

(Source: Shogakukan, translated)