Cheonssisegaui Mangnaniga Doeeotda

There was a nameless warrior who devoted his life to guarding and serving Cheon Mu-ryang, the eldest young master of the Cheon Clan. In the end, he was betrayed by Cheon Mu-ryang and met his death. But when he opened his eyes again, he found himself reborn as the very man who killed him: Cheon Mu-ryang, the infamous mad dog and eldest son of the Cheon Clan. With his memories of the past, his martial arts, and a cold, calculating judgment, he dons the mask of a scoundrel and steps into the clan’s internal struggles for power. From within, he begins to save the Cheon Clan from destruction... To rewrite the future itself.

(Source: Naver, translated)