Mordred
Mordred in one legend was known to be the thief of king Arthur's second sword Clarent, which was known as Arthur's sword of peace. This being the reason why it is not as well known as Excalibur, which was Arthur's sword of war that was used to protect of Camelot. Clarent was a sword that would be present whenever Arthur would make a covenant of peace. The exception to the usually peaceful status of Clarent being the legend in which Mordred stole it and later used it to mortally wound Arthur just before being cut down in turn by Arthur.