Impel Down, located in the Calm Belt, is known as the world's greatest prison in the world of One Piece.
The prison is divided into six levels, or perhaps it's more like six and a half, if you count Newkama Land. Countless criminals are locked away here, among them some of the most infamous pirates in the world, figures who could stand alongside legends like Whitebeard and the Pirate King, Gol D. Roger.
A fortress designed to hold such dangerous individuals naturally has formidable defenses. It wouldn't be known as the "Iron-Walled Fortress" for nothing. Its location in the Calm Belt, a natural nest for Sea Kings, makes any approach or escape incredibly difficult. At the same time, Impel Down forms a triangle with Marine Headquarters and Enies Lobby, allowing for rapid support in the event of a prison break.
To top it all off, the Warden of Impel Down is said to possess power comparable to that of a Marine Admiral.
In the prison's entire history, only one man has ever successfully escaped, Shiki the Golden Lion. And even he had to leave his own two feet behind to do it.
...
"How the hell did I end up in the world of One Piece? And locked up in Impel Down, of all places?"
Kai stared down at the black-and-white striped uniform he was wearing. After piecing together what he could from his "fellow inmates," he collapsed onto the floor, feeling like his life had lost all meaning.
He remembered it clearly. He had been a perfectly content couch potato, enjoying the air conditioning while playing a One Piece mobile game. He'd even spent a dollar on a cheat program. Just as he used it to defeat a boss and get a god-tier item, he was hit with a ban.
Furious, he had slammed his phone on the table.
And then... there was no 'and then'.
When he woke up, he discovered his new name was "Cori Yekai."
'What kind of name is that?' he thought. The last part, Yekai, is just a weird version of my old name. He decided he would just go by Kai from now on.
His t-shirt had been replaced by a striped prison uniform, and his air-conditioned room was now a cell with iron bars. The one silver lining was that he had a private cell. All around him were similar cells, each packed with several burly men in the same uniform.
When Kai first woke up, surrounded by inmates built like bulls, he thought he'd been thrown into a foreign prison and immediately feared for his life.
Only after calming down did he realize he wasn't himself anymore. He still had black hair and Asian features, and his face was mostly the same, just more chiseled and defined. He was also now two meters tall. The overall feeling was that he was an enlarged, more handsome version of his original self.
He quickly understood this wasn't a simple case of his soul being transported. His entire body had been pulled into the world of One Piece, just altered in the process.
He learned the truth from the inmates in the cell across from his. This wasn't some foreign jail, it was the world's number one prison from One Piece, Impel Down! Looking around again, he saw that the walls were carved from massive stone blocks. The style was ancient and lacked any modern touch, confirming it was nothing like a prison from Earth.
"Impel Down..." he muttered. "The greatest prison in the world? How in the world did I end up here?"
Kai desperately hoped it was all a dream.
Unfortunately, it wasn't.
He slammed his fists against the wall and the iron door. The sharp pain that shot through his hands was a clear reminder that this was all real. He was truly in the world of One Piece.
On the surface, the death rate in One Piece didn't seem as high as in a world like Naruto. But that was only because the story followed the perspective of Luffy and his crew. For everyone else, the world was filled with pirates who pillaged and murdered without a second thought. The power levels here were also terrifyingly high, with absurdly broken things like Devil Fruits.
"So, what's my cheat? My system?"
"System? You there?"
"System, wake up! Give me my newbie reward! Give me a mission!"
"Damn it... there's no system?"
"What about a Devil Fruit? Any special powers or a unique bloodline?"
"..."
After muttering to himself like a madman, Kai came to a grim conclusion.
He had no system.
He had no special abilities or bloodline.
And he definitely had no Devil Fruit.
"I'm completely useless..."
Kai felt his hope drain away. Without any kind of advantage, what could he possibly do in a world like this? Become cannon fodder? Worse, his starting point was Impel Down. Escaping without power was impossible. The death rate inside was notoriously high, and even though he was only on Level 1, the Crimson Hell, he knew it was still a dangerous place.
"Have any of you heard of a pirate named Luffy?" Kai shouted to the other prisoners, hoping to get a sense of the current timeline. If he knew when Luffy would break into the prison, maybe he could tag along and escape.
The chances were incredibly low, he knew. When Luffy eventually stormed Impel Down to save Ace, only the strong managed to escape with him. The weak were either left behind or became collateral damage in the chaos.
In the end, everything came down to strength.
From the cell opposite his, which held seven or eight other men, a voice called back.
"Luffy? Never heard of him...!"