Having acquired a cheat abilitie , I became unrivaled

Tenjou Yuuya has been a bullied boy since the past. He lives in the home of his beloved grandfather while he goes to school. As usual, he receives a harsh bullying, and he takes an extended absence from school to have some time to heal his wounds. While on this long time of absence, he takes the opportunity to clean his grandfather's house and goes to a room he never went before, where his grandfather keep many different objects that he gathered from his travels around the world. While he was arranging the objects, he found a door not on a wall among the objects. Out of curiosity he opens this door, what he found on the other side was... I'm only translating the original: https://kakuyomu.jp/works/1177354054882725960