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 the first dungeon, Shijima Cave, located near the hero's base. Finding the dungeon underutilized and filled with inactive monsters, Nargoa initiates significant changes. After a year, the once-ineffective dungeon becomes a formidable labyrinth, too challenging even for mid-level hero groups. However, Nargoa faces and defeats the legendary heroes who had already vanquished the Demon King, risking the safety of its monster staff. In an unexpected turn, Nargoa is reborn into the world a century later. In this new era, a story unfolds involving the Shijima Cave, now a renowned labyrinth, and its monster employees.