There are two characters whose qualities and actions involve using supernatural powers or magic to affect the fates of other characters: Merlin and Morgan le Fay.
Merlin is a wizard who can control various animals and natural phenomena. But it is his use of magic to disguise Uther Pendragon as the Duke of Cornwall in order to deceive the Duke's wife that sets in motion the legend of King Arthur. Uther impregnates Igraine, the Duke's wife, and she gives birth to Arthur.
Uther, during a battle over England's sovereignty, declares than only a king will bear his magical sword Excalibur, believed to be forged from magic also. He plunges the sword into a stone (also a magical act) and it remains there for years while the land, without a king, becomes desolate and disease-ridden. Despite being illegitimate, Arthur wins his right to the throne of England because he is able to free the sword from the stone.
Years later, Arthur's half-sister, Morgan le Fay, disguises herself as Arthur's wife Guinevere, and seduces him into impregnating her with Mordred, whose destiny is to kill his own father. These acts of supernatural magic directly impact the outcome of the story.