Hwansaengcheonma

Regret is the bane of old men. When aging Alliance Lord Hajin Cheon of the Grand Martial Alliance reflects on his immense legacy, he finds that he recalls little of his deeds or even the lives of his closest advisors. Stricken by a mysterious ailment, Hajin suddenly passes away only to wake up in the body of Ridan Byeok, heir to the Byeok Clan of swordsmen. The only problem is, Ridan is a well-known wastrel, the polar opposite of the virtuous and stern Hajin. Righting the wrongs of his predecessor, Hajin begins to uncover the truth behind his death…

(Source: WEBTOON)