Rebirth in One Piece as Deputy Captain - Chapter 306

Xuanyuan was reborn into a strange new world where training to be a Xian was a cornerstone. There was however something ancient dwelling inside his body. He could now devour all of creation…

This is the journey of an ordinary boy and his transformation into the supreme being that reigns over all existence! Everywhere he went, he could not help but tie his fate with numerous women. It did not matter whether they were a goddess or a devil…

- Description from Novelupdates

A skilled but unfortunate modern-day assassin was killed in a freak accident. However, he soon found himself summoned to another world by an evil wizard, who had him possess the body of a boy as an experiment. After successfully planning the evil wizard’s downfall, he used the wizard’s body as a springboard to gain influence and power in his new world.

- Description from Novelupdates

Mikail Fremawan, someone who has always been followed by bad luck since he was a child, and the peak was when both of his parents died simultaneously before his own eyes.But the sudden appearance of the System changes everything, from wealth, power, glory, and... revenge; Mikail begins his steps to achieve all of that one by one.Witness his story of turning his misfortune into incredible good fortune.Mikail: There might be something wrong with my System, Isn't it?

Beings that create and control monsters and demons, beings that build and reign over malicious labyrinths, the chosen individuals to possess an overwhelming Unique Skill… They are the Demon Lords.