Alchemy of Souls

In the fictional country of Daeho, young mages struggle to overcome twisted fates caused by the forbidden "alchemy of souls," a spell that swaps souls between bodies. Naksu, an elite assassin, finds her soul trapped in the weak body of Mu Deok, a servant to the noble Jang Family. Jang Uk, the young master of the Jang Family, harbors a dark secret about his birth and seeks Naksu's help to change his destiny. (Source: MyDramaList)