MTL - The Whole Live Room is Knocking Me and the Female Partner’s CP - Chapter 15 saintess

Turned into a vampire with no political backing, wealth, or power, Dylan immediately tries to become the strongest there is. Facing numerous dungeon breaks and a coming invasion of unknown origin, Dylan must do everything he can to fight back. In the process, Dylan tries to make the world a better place in his way, by taking control of as much land and women as he can. Watch as Dylan amasses power and influence among monsters and humans and rises to the top in more ways than one.

He is a young man without a spirit root. It is believed that this denies him the chance at cultivation. However, a mysterious lady imparts a Supreme Demon Classic to him and from then on, he starts his path of cultivation.
He undergoes a total transformation to emerge as the most fearsome and powerful fiendish demon in his era, that even immortals and fiends are fearful of him, and saints are at his beck and call.
Mastering the Supreme Demon Classic is the turning point in his life. He is able to unleash his immense and divine power from within, illuminating the entire universe!

- Description from Novelupdates

“Future Occupation…Demon King.”

Sai Akuto enters Constant Magic Academy to be useful to society and to become a top-ranked national magician. However, a horrible prediction is made on his first day. Thanks to this prediction, the class representative girl hates him, a girl with a mysterious power embraces him, a female artificial human is sent by the empire to observe him, and his school life is otherwise thrown into chaos…

Crossing to One Piece world, he became Roger’s younger brother.