Seian

Kai, the Grim Reaper who guides the souls of the dead. After completing his ten thousandth mission, he chooses extinction when the god of death, Shinos, offers him one wish. However, Kai is reincarnated as Seian, the spoiled second son of the Schreyton Counts. What will become of Kai's fate, now that he has taken on Seian's twisted life?

(Source: Kakaopage, translated)