The Regressed Son of a Duke is an Assassin

Alternative:

The Reborn Young Lord Is an Assassin / The Regressed Youngest Son of the Duke is an Assassin / 회귀한 공작가의 막내도련님은 암살자

Cyan Vert, the 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!