10 nen goshi no HikiNiito o Yamete Gaishutsushitara Jitaku goto Isekai ni Ten'ishiteta - Volume 10, Chapter 10: Bulletin board Yuuji, reporting the fact that he would likely obtain large amounts of money

I thought I had seen bad endings before—but nothing prepared me for the garbage conclusion of The Hero’s Path.Years of investment, countless struggles, and for what? The protagonist failed. The villain won. Everyone else? Dead. Furious, I sent a message to the author, only to get a chilling response:[Do you think you could do better?]Before I could even blink, my phone exploded in a blinding light, and the next thing I knew—I was waking up in a completely different body.Now, I’m Merlin Everhart, a fourteen-year-old with a 12-star talent, the kind of gift only the strongest should have.But here’s the problem: I have no idea who I am supposed to be.With nothing but a system and a first-day-of-school alarm ringing in my ears, I’m thrown into the world of The Hero’s Path.Am I just another side character in a world that already ended once?…Either way, I refuse to let this story beat me too.

Where there is light there shall be darkness

Hero? Nahh I'm the demon.

In a world where the demons are looked on as the incarnation of evil, the human-demon war raged on across the whole continent.

The Seraphims, protector of humanity, and the Demon lords who rule across all demonkind. Amidst all the chaos, Simon was reincarnated as the lowest ranking demon noble.

A brutal world where the strong feeds on the weak, eat or be eaten, kill or be killed. A demon far off from humanity….a dream unfulfilled in his previous life will be achieved in this life. Taking on the moniker of Laplace, he destroys all his enemies and rise over them.

He rose from abjection and achieved eternity in glory.
He embraced the prettiest beauties and killed the strongest opponents.
He was able to shift the clouds into rain and cover the sky with one hand.
He could overrun the entire world and go anywhere as he pleased.
He pacified wars and held the world in awe.
He was matchless in the world.
However, his hair turned white overnight for his beloved woman and he was on the verge of death.
He was surrounded by enemies who were trying to take his life.
He struggled his way to become mighty and fought for everything he cherished.
For this age, he was the only king of the Dark World.
His name was a byword for might.
As a man living in the world, his only ideal was to pursue brilliance.

- Description from Novelupdates

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.