MMORPG: The Elementalist - Chapter 621

Amidst an ordinary stroll home, Jake is struck on the head by a mysterious object, plunging him into unconsciousness. When he opens his eyes, the world has transformed. Now, reborn into a newborn in a magical realm, Jake grapples with the surreal fusion of past and present. Navigating the challenges of infancy, he discovers latent magical abilities tied to this extraordinary reincarnation. As the threads of his former life intertwine with his newfound existence, Jake must unravel the mysteries of his past to fulfill a destiny that spans realms and confront the enchanting and perilous journey that awaits him.

I became the Heavenly Demon’s dog.

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.