The Shadows of Who We Once Were Vol. 2 (Kodansha, 2022)

When the 27 students who graduated from class 6-2 of Shinozuka Elementary gather for their class reunion years later, they were locked in the now-abandoned school by their classmate, Mikio Yumesaki. He conducts trials and experiments, all for the sake of ""testing one's goodness under extreme circumstances."" 26 high school students and one teacher are locked up in an abandoned school building. The culprit is Mikio Yumesaki, a psychopath trying to ""experiment"" on the goodness of people's hearts and the weakness in their will. No one saw it coming when one of the 27 turned on their allies and killed a student. Cornered and scared, the students decided to take justice into their own hands, but the outcome was something no one would have been able to predict.