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

"Dungeons emerged in the world five years ago. These mysterious spaces are off-limits to police and military forces, accessible only by individuals aged 15 and older through their own free will. Inside, people can level up, acquire fantastical items, and for some, undergo transformations that render them astonishingly beautiful. Yuuta Rokujou has endured bullying since kindergarten, leading him to avoid attending the popular Dungeon High School. Instead, on his fifteenth birthday, he chose to venture into a dungeon alone. This is the story of how Yuuta, under such circumstances, grew stronger than anyone else, eventually becoming one of the twelve extraordinary individuals known as the 'Twelve Heroes,' capable of influencing the fate of the world." -- From MangaDex