The Regressed Son of a Duke is an Assassin is a Manga/Manhwa/Manhua in english language, Action series, english chapters have been translated and you can read them here. You are reading chapters on www.mgeko.cc fastest updating comic site. The Summary is

Cyan Vert, the best assassin of the continent, meets a pitiful death after having been betrayed by his own brother, whom he had trusted all his life. If I were given another chance at life, I would live it differently. I would only trust myself, and achieve all the things I want on my own without serving anyone else but myself. That is how I was given a second chance at life. The Cyan Vert, a shadow who lived for others, is no more. I will now pave a path on my own, for myself!