From Welsh
Medraut, meaning uncertain. In Arthurian legend Mordred was the illegitimate son (in some versions nephew) of King
Arthur. Mordred first appears briefly (as
Medraut) in the 10th-century 'Annales Cambriae', but he was not portrayed as a traitor until the chronicles of the 12th-century Geoffrey of Monmouth. While Arthur is away he seduces his wife
Guinevere and declares himself king. This prompts the battle of Camlann, which leads to the deaths of both Mordred and Arthur.