Descendant of the Sun (1983)

Yuen of the celestial has gathered sun light and practiced for 500 years, and now becomes immortal. He descends to the world to prevent the black snake monster from scourging. The insidious acting king sends people to murder the virtuous princess in order to grasp at authority by himself, but the murderers are defeated by Yuen. So the acting king invites the black snake monster who transformed to human form, to fight Yuen. The well-matched two knock the hell of each other and come to even.