In Malory’s Le Morte d’Arthur, Sir Accolon of Gaul awoke after going to sleep on an enchanted ship, and
he found himself by a deep well-side, within a half a foot, in great peril of death. And there came out of that fountain a pipe of silver, and out of that ... ran water all on high in a stone of marble.
In some versions, while Accolon is at the fountain, a lady appears to him and mistakes him for King Arthur. The lady tells Accolon that she has been wronged by a knight (who is actually Arthur in disguise) and that she seeks vengeance. Believing that Accolon is Arthur, she encourages him to challenge the knight to a duel.
Accolon agrees to the duel, unaware that he is actually facing his own king. The duel takes place, and Accolon fights valiantly against Arthur, not knowing the true identity of his opponent. During their duel, the lady’s true intentions are revealed, and Accolon learns the truth about his opponent’s identity. The duel is stopped, and Accolon’s loyalty to Arthur is reaffirmed.
The lady is Morgan le Fay in disguise. This event serves as a demonstration of Morgan’s cunning and magical abilities, as well as her ongoing attempts to undermine King Arthur and his court.
Source
Le Morte Darthur | Sir Thomas Malory, 1469-1470