Dungeon ga Arawarete 5-nen, 15-sai de Dungeon ni Idomu Koto ni Shita.

Five years ago, mysterious dungeons suddenly materialized across the world. Neither law enforcement nor military forces can breach their boundaries; only individuals who have reached the age of fifteen are permitted to enter, and solely by their own free will. Within these enigmatic labyrinths, adventurers can increase their strength, obtain fantastical equipment, and even undergo remarkable physical transformations that grant them extraordinary beauty. Yuuta Rokujou has endured relentless bullying since his earliest days in kindergarten. Because of this painful history, he refuses to follow his peers into Dungeon High School, the specialized institution where most teenagers prepare for exploration. Instead, on his fifteenth birthday, Yuuta makes a bold decision: he will step into a dungeon entirely alone, seeking to change his fate through his own power.