The Yu Brother's Case Book - Volume 2 Chapter 0-1

He died at twenty-two—bitter, brilliant, and broken. But death wasn’t the end.Now, Noel wakes up inside a tragic fantasy novel he read long ago... as a nameless extra doomed to die before chapter five. But there’s a catch—this “extra” isn’t ordinary.Armed with future knowledge, a sharp tongue, and a magic talent that defies common sense, Noel won’t just survive.He’ll rewrite the story.Because in this world?The real genius never had a name—until now.

Nicholas Marjan—known as Niels—was once an Asian football-obsessed loner in the modern world. A master of FIFA Career Mode, addicted to tactical YouTube breakdowns, and endlessly chasing transfer window drama, he lived the beautiful game from behind a screen.Until the day he died.Reborn in the late 2000s in a small European country, Niels wakes up in the body of a once-promising young footballer whose career was tragically cut short by a brutal ACL injury. With no chance to play again, and haunted by the weight of lost potential, the only path forward is through coaching.Now an assistant at a struggling lower-league club, Niels carries two things no one else has:-Knowledge of football’s future-A mysterious cheat-like instinct for scouting raw, hidden talentAs he begins to make waves with his unique coaching style, Niels catches the attention of the football world. Offers from top-tier teams in the top five European leagues come rolling in. With a growing reputation, Niels sets out to transform not just one team's fate—but to leave his mark on the biggest stages in world football.

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

Another entry in what you can call the Proto Harem Series: the books written before the Harem Series that take place in the same world.

It’s about a swordswoman whose student is captured, so she goes to rescue her.