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. This room holds a mysterious power that changes his life forever.