In his previous life, Qiao Yinan died at the hands of the biggest BOSS and was forcibly assimilated into a monster, causing his soul to collapse. After regaining his life, he vowed to complete the killing mission in his previous life.
Generally speaking, this is the story of a person who was reborn and decided to refresh the dungeon with hatred. He went back to five years ago and accidentally picked up a powerful teammate on the way and killed the BOSS.
(All inspiration comes from "Bloodline")