King Arthur: have you heard of him? He was an awesome king who basically invented chivalry and recruited a bunch of noble knights to bring justice to England.
Trouble came, though, when his best knight, Lancelot, and his gorgeous wife, Guinevere, fell in love. In many versions of the story, the affair that resulted ended up destroying Arthur, the Knights of the Round Table, and Camelot itself.
For more on this tale, check out Le Morte D'Arthur by Sir Thomas Malory.