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

Synopsis:

Five years ago, dungeons suddenly appeared across the world. These mysterious spaces are inaccessible to the police and military, and only individuals aged 15 and older may enter—by their own free will. Inside, adventurers can level up, obtain powerful fantasy items, and in some cases undergo transformations so extreme they become astonishingly beautiful. Yuuta Rokujou has endured relentless bullying since kindergarten. Because of this, he chooses not to attend the Dungeon High School everyone else enrolls in. Instead, on his fifteenth birthday, Yuuta makes a bold and dangerous decision—to enter a dungeon alone, challenging both his fate and the world that has always rejected him.