Throne of Fire (1983) on Netflix

A mighty hero battles the son of Satan and his evil witch ally to save a kingdom from being taken over by the duo.

On a terrible night, a woman gives birth to the grotesque abomination, Morak, after her unholy union with the powerful demon and Hell's messenger, Belial. To become the king of the world and finally sit on the infernal Throne of Fire, Morak must first kill the benevolent rightful ruler, usurp the throne during a total lunar eclipse, and forcefully take the fair Princess Valkari as his wife. Now, only Siegfried, the mighty barbarian warrior with the extraordinary power of invisibility, can save the innocent noblewoman and the entire humankind from a fate worse than death. Will he fulfil his destiny, and destroy the son of evil?

—Nick Riganas

Standard muscleman fantasy finds Siegfreid fighting evil personified in the guise of Morak. Morak is the son of the devil's messenger, Belial. Morak is attempting to take the throne of evil "on the day of the night in the day", or rather during an eclipse. To escape instant death by flames, before sitting on the throne, he must also wed Valkari. Siegfried who is invulnerable except to fire and gains temporary powers of invisibility (per "The Rings of Nibelung"), of course, battles Morak to save the beloved princess.

—John Sacksteder <jsackste@bellsouth.net>