King Arthur's illegitimate son, Mordred, betrayed his father by raising his sword in battle against him.
- In The Inferno, Mordred is mentioned very briefly as Dante and Virgil travel through Cocytus' first round, Caina, which is reserved for those who've committed treachery against members of their own family.
- In early Arthurian legend, Mordred was originally Arthur's nephew, not his son; the child of King Lot and one of Arthur's half-sisters, either Morgan le Fay or Morgause/Anna. He was also the half brother of Sir Gawain (who defeated the Green Knight through his bravery and honesty), and Lady Elaine (the wife of Lancelot and mother of Sir Galahad). Mordred became a knight of the Round Table, but when Arthur was away at war, Mordred seized the throne of Camelot and seduced Arthur's queen, Guinevere. When Arthur returned, Mordred waged war against his uncle. In battle, although Mordred succeeded in mortally wounding Arthur, the king killed his treacherous nephew.
- Later versions make Arthur the father of Mordred, when the king is tricked into having an incestuous affair with Morgause. Arthur has a vision of his death at Mordred's hands, yet allows his son to be trained as a knight. Mordred's connection to Guinevere is omitted (she instead is seduced by Lancelot), but her adultery causes Arthur's knights to turn on one another. Mordred seizes this chance to claim power in Camelot and use it to finally destroy Arthur.