MTL - My Doomsday Territory (My Apocalypse Domain) - Chapter 8 call

A girl who calls herself a demon 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 “summon 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 a false 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.

Void plane: synonymous with terror and taboo, no one knows how big the void is, because it is growing every moment, and anyone who talks about the void will be shocked.

Void Lords: The voices of the Void plane, the plane’s invaders, conquerors and destroyers; they lead the Void’s armies all the way to war, and when they discover a new plane, they will do everything possible to pull it into the Void embraced by the Void’s reward.

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- Description from novelbuddy

Sam Sanderson is envied by many for his job, however he himself didn't seem to like it much.

Due to his expertise in nanotechnology he was tasked with monitoring the state the Valkyries, humanity's strongest weapons who all just happen to be made into gorgeous females.

Forbidden from even touching any of them, Sam spends his days in agony watching flowers he couldn't hope to touch. However one day all of that seemed to change.