The Legend of King Arthur (1979)

Dark Ages wizard Merlin, weary of the barbarism around him, creates a new order of enlightenment and justice with a youthful Arthur at its head. Merlin gifts Arthur with the magic legendary sword Excalibur to help him defeat the nobles who oppose his rule. But Arthur must also beware his half-sister Morgan, a sorceress who has sworn to kill him to avenge her father's death. As Morgan intensifies her plans for revenge, she uses magic to draw Lancelot and Guinevere into a passionate affair. However, it is the still more traitorous Mordred who will ultimately determine the fate of Arthur's rule.