SSS-RANK SUMMONING WEAPON SYSTEM

Xavier Reaper, a high school student, was on a school trip with his classmates, when they encountered a female corpse in the middle of the deserted road. After discussing, or rather, complaining, they decide to move the body to the side of the road so that they can continue on their way.

But they're in for a surprise, as Xavier goes to move the body, the corpse glows and suddenly it explodes, taking the whole of the school bus and his classmates.

Not knowing what happened, Xavier finds himself chained, filthy, and looking like he hadn't bathed in a month, being transported somewhere in an equally filthy carriage with his classmates.

What is going on?

(The R-18 is gore scenes, so please be informed. )