I got cheat skill and became unrivaled in the real world

Tenjou Yuuya- seen as a ugly freak due to his overweight and ugly physique was bullied since he was young. Now he lives in the home of his beloved grandfather while he goes to school. As usual, he receives a harsh bullying.
Finally graduating from High School, he takes it apon to better himself, starting by cleaning his grandfather's house and goes to a room he hasn't seen what feels like years, 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...