Isekai de Cheat Skill o Te ni Shita Ore wa, Genjitsu Sekai o mo Musou Suru - Level Up wa Jinsei o Kaeta

Since a long time ago, Tenjo Yuuta has always been bullied. He goes to high school while living alone at his late grandfather’s house. Although he was always met with severe bullying, he found solace during the long holiday break.
During this time, while cleaning his grandfather’s house, he also decided to clean the room where his grandfather keeps the items he collected throughout his life. A room where Yuuta has never entered before. While trying to organize the items collected by his grandfather, he found a mysterious door that was seemingly cut out from the wall.
Out of curiosity, he opened it. And then...