MC has been fighting the 10 corpses (high ranking demons that are conspiring to destroy humanity)
One of the 10 demons has activated an "artifact" that pulled a lot of kids (MC included) into a tower/dungeon.
They can only leave after clearing floor 100. However they where all nerfed to level 1 (1% of their natural power).
Last important point, if you get extra strong inside the tower/dungeon you get to keep that strength outside so MC is taking the opportunity to farm!
Don't mind me, I'm just alive until I die