No.6, book 7
Atsuko Asano et Hinoki Kino2014

SHION! WHAT ARE YOU TRYING TO DO?!
Safu is a prisoner on the top floor of the Correctional Facility. Shion and Rat manage to slip back in amid the chaos sown by Dogkeeper, but what they find is almost too terrifying to conceive… countless neatly organized rows of corpses, with their brains removed! Backed into a corner, Rat is ready for death, but Shion seizes a weapon, and insists on surviving at any cost—!