Bu Sha - Volume 6, Chapter 4: Mystery of the Past

[You have entered the dungeon at Gwachun Station’s 1st Exit.] When former high school student Kang Woojin finds himself returned back to Earth after being forcibly summoned to a foreign planet for 20 years, he soon finds that Earth is not the same, normal place as he once remembered it to be.

“Mr. Qi Lin, may I ask how you thought about joining the entertainment circle from a sports student?”

“Uh, I said that I was there to deliver lunch boxes, do you believe me?”

- Description from novelbuddy

A 34-year-old NEET otaku, chased out from his house by his family, found that his life is reaching a dead end. He then recalled that his life could actually have been much better if he had made better choices in the past.

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.