Nakabosu-san Level 99, Saikyou no Bukatachi to Tomo ni Nishuume Totsunyuu!

A monster named Ichi Monster Nargoa is assigned as the middle boss of Shijima Cave, the first dungeon near the heroes' base. Finding the dungeon underperforming with only unconscious monsters, Nargoa initiates major changes. A year later, the dungeon becomes so formidable that even mid-level brave parties reject it, transforming it into a legendary labyrinth. After Nargoa defeats the Demon King and challenges individual heroes to protect its inhabitants, he somehow reincarnates into the world a century later. In this new era, a story unfolds involving the Shijima Cave and its employees, now part of a renowned and treacherous labyrinth.