With My Extraordinary Classmates in Tower

A historic event involving 51 universes, occurred a thousand years ago. All races, across 51 universes, fought against one catastrophe - Gods, giants, demons, humans. In its absence there was no form, no body, no emotion. It was an entity created only to bring death: ENTITY;DEATH. It was chaos. The allied forces of 50 universes managed to kill this individual, but at what sacrifices? Death of many friends, lovers and acquaintances. Just as they were to celebrate their victory against one single being, they found out there were 499 more units that are similar to ENTITY;DEATH. Many called this event Ragnarok Thousand years later, at the present, a class of eight people was transported to a certain Tower.