One Piece: My Days as a Prisoner in Impel Down

Luo En Black, a third-class Marine, had spent twenty years in the service without a single achievement. Everyone in his battalion considered him useless.

No one knew he had eaten a Devil Fruit labeled "useless" in the encyclopedia: the Cage-Cage Fruit, which made him stronger the longer he was confined in an enclosed space.

He had just one goal in life: move into the underwater prison Impel Down and quietly keep his head down for the rest of his days.

But the more he tried to hide, the less the world would let him. During recruit training, his instructor forced him to leave the Marines. During his reassignment, Akainu drove him onto the battlefield. Just when he finally managed to get into Impel Down as a guard, Luffy's prison break, Blackbeard's invasion, and the call-up for the Paramount War dragged him out of his comfort zone again and again.

"I just want to sit in prison. Why is that so hard?"

The day he finally entered Level 6 as an inmate, the entire floor's worst criminals came to "welcome" him. He had them all embedded in the walls within five minutes.

"Can you all stop making a fuss? I want to sleep."

This is the absurd story of a man who gets stronger the longer he stays in prison, using the art of keeping his head down to fight the entire world.