Emperor Charles, The Siege of Vienna.
Rome, circa 1556

The evil son of Arthur and his half-sister Morgana, Mordred seizes the throne of England when Arthur, Gawain, and their troops pursue Lancelot into France. Himself the child of incest, Mordred attempts to claim Guinevere, Arthur's Queen, as his own wife in Arthur's absence. Taking Guinevere into Winchester, Mordred "said plainly that he would wed her (which was his uncle's wife and his father's wife)," as Malory explains in his Morte DArthur. Guinevere, however, cleverly resists, saying she must go to London to shop for their wedding. Once there, she takes refuge in the Tower of London, providing it "with all manner victual," as Malory says, and surrounding it with an army which successfully fights off Mordred's attacks. The Bishop of Canterbury, at this point, tries to reason with Mordred in a speech which again, seemingly unwittingly, points up the incest theme. He chastises Mordred for his immorality in this immortal speech:

"Sir, what will ye do? Will ye first displease God and sithen shame yourself and all knighthood? For is not king Arthur your uncle, and no farther but your mother's brother, and upon her he himself begat you, upon his own sister? Therefore how may ye wed your own father's wife? And therefore, sir,...leave this opinion, other else I shall curse you with book, bell, and candle."

Incest for the medieval audience was strictly taboo, extending not only to cousins and distant relations but to godparents of the same child. Mordred, the child of incest, is absolutely evil, a villain and traitor who mortally wounds his father in the final battle in the same moment that Arthur kills him.