Dungeon Ga Arawarete 5-Nen, 15-Sai De Dungeon Ni Idomu Koto Ni Shita. summary:

Dungeons appeared in the world five years ago. Police and military are unable to enter these enigmatic spaces, and entry is permitted solely by the free will of individuals aged 15 and above. Leveling up is possible, fantasy items can be acquired, and for some, the transformation can lead to becoming astonishingly beautiful.Yuuta Rokujou has lived a life of being bullied since his kindergarten days. Due to this, he decided not to attend Dungeon High School that everyone else went to, rather, on his fifteenth birthday, he decided to challenge a dungeon alone. This story is about how Yuuta, under such circumstances, became stronger than anyone else, becoming one of the twelve individuals capable of moving the world, known as the ‘Twelve Heroes.’