Die Letzte Rache (1982)

This 1982 film tells the story of the a man, who searches for an ancestor for an aging ruler. The man doesn't find a dignified heir, and he is tempted to seize power on his own. But the ruler foils the plan and uses the him to gain immortality in a science experiment. The man then takes revenge on him and destroys the rulers empire. Then, the son and daughter of the ruler, who incestuously love each other, take revenge for the withholded heritage: they torture their father in his immortality.