The Reborn Young Lord is an Assassin

Cyan Vert, illegitimate son of the Duke and the empire's greatest assassin, is betrayed by the righteous brother whose shadow he's always lived in. But just before the fatal blow strikes his throat, Cyan awakes to find he's just a boy again. The young lord has been reborn, and this time, he's not going to live in anyone's shadow!

(Source: WEBTOONS)