A brief description of the manga Confinement King:
A girl who calls herself a devil appeared before a timid student named Fumio Kijima. The girl claims to be a campaign girl from the demon world and she gives him the ability to “create a room” from nothing but thin air.
At first he doesn’t abuse his power and uses it as a convenient ability in his every day life. But one day he is bullied by his classmates under an unfair pretext and is made to suffer. In the past he would have wallowed in misery and cried himself to sleep, but now he had a power to call upon. “I will make them regret it!”
Using his powers to summon his “room”, he begins his era of confinement and brainwashing to achieve his goals.