I Got a Cheat Skill in Another World and Became Unrivaled in the Real World, Too

Since a young age, Tenjou Yuuya was continuously bullied. He is staying in his beloved grandfather's home while he goes to school and, as usual, he receives a harsh bullying, and takes an extended absence from school in order 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 finds a room he never had seen before. Due to this being where his grandfather kept many different objects that he gathered from his travels around the world, it needed tidying! While Tenjou was arranging the objects, he found a door, yet it didn't seem to go anywhere since it was standing upright and not attached to any wall. Out of curiosity, Tenjou opens this door, and what he found on the other side is...