mc is going to "die" in the dungeon during a fight (blasted off a bridge by a jealous classmate he gets trapped DEEP in the dungeon and adapts to eating monster flesh/getting their powers via a milk thing he finds) and that is where he gets his power up,
book 1 deals with the xfer, them leveling, him "dying" and using his power to adapt/get powers then he is a typical OP mc style harem. end of book 1 is him meeting the vampire girl at the bottom of the dungeon and escaping the dungeon, book 2 is bunny girl who is super strong and can see the future. he really does not meet up with his classmates again for several books