Myst, Might, Mayhem

Jeong, the Slaughtering Scythe Demon, feels joy in killing others. He happens to look just like Mok Gyeongwoon, the third young master of the Mok Sword Manor...

"Be my substitute, and I'll let you out of prison."
"That opportunity. Will you give it to me?"
Crack!!!

He kills the Third Young Master and decides to become the real one. Can the fake Mok Gyeongwoon survive as the real one? The serial killer begins his new life as the Third Young Master of the Great Mok Sword Manor.

(Source: Naver, translated)