So Ji-sub

Gurus in the late Goryeo dynasty try to obtain a fabled, holy sword, and humans in 2022 hunt down an alien prisoner that is locked in a human's body. The two parties cross paths when a time-traveling portal opens up.

A contract killer becomes disenchanted with his line of work and spares the lives of his two latest would-be-victims. But the assassination company he works for has no intentions of letting him quit.