Yoon Je Moon

Yoon Hyun Woo is a devoted, hard-working, and loyal secretary to the Soonyang chaebol family, which runs the lucrative Soonyang Group business empire, founded by Jin Yang Cheol. During a power struggle between Jin Yang Cheol's children for control of the empire, Hyun Woo is ruthlessly betrayed and killed under the orders of an unknown member of the chaebol family. Miraculously he wakes up as a kid inside the body of the youngest member of the family – Jin Do Jun. After he realizes what has happened, Yoon Hyun Wo plots to use his new “identity” to formulate a hostile takeover of the group – and punish the people who killed him. But, will the cold-blooded and money-hungry CEO of Soonyang Group, one who firmly believes in right of primogeniture, be so easy to win over or conquer? Will Hyun Woo be able to wrest the reins from Jin Yang Cheol's children? And what role will be played in all this by a dogged public prosecutor named Seo Min Young, nicknamed “Soonyang Group Grim Reaper”? (Source: LuckyGirlGwenIce at MyDramaList) ~~ Adapted from the web novel "Youngest Son of a Conglomerate" (재벌집 막내아들) by San Gyung (산경).

Early in his reign, King Se Jong accidentally causes the death of his in-laws and their slaves in an attempt to save them from his brutal father, King Tae Jong. Two young slaves, Ddol Bok and his friend Dam, survive, but neither knows the other has. Ddol Bok blames the king and vows to kill him to avenge the deaths of everyone he loves. He spends several years becoming a great warrior. Dam, who has a photographic memory, becomes a court lady called So I, but she feels so guilty about her role in the other slaves' deaths that she is mute. Ddol Bok returns to the palace under the name of Kang Chae Yun in order to kill the king. However, he becomes embroiled in the mystery surrounding the deaths of several Hall of Worthies scholars. The deaths were caused by a secret society called "Hidden Root", which was created long ago by followers of Jung Do Jeon, with the goal of giving more power to ministers and less to the king. King Tae Jong killed Jung Do Jeon and all of his family except his nephew, Jung Gi Jun, who is now the leader of Hidden Root and lives in disguise not far from King Se Jong.