Mordred (Modred, Medrawd, or Medraut) has become the traitor and the villain in the King Arthur legends. Described as Arthur's nephew by some, while many stories cite Mordred as being Arthur's bastard son by his half-sister Morgause, the wife of King Lot.
The Annales Cambriae mention battle of Camlann, "in which Arthur and Medraut fell" in the year 537, and gives no details about their relationship, nor on whether they were fighting on the same side. In another Welsh story, the Dream of Rhonabwy, war breaks out between Arthur and his nephew Medrawd because of a messenger, Iddawg, delivering Arthur's request for peace in the "rudest possible way," hence causing war.
Geoffrey of Monmouth In the Historia Regnum Brittonum of 1136, is the first to make Mordred the traitor who causes the downfall of Camelot and the death of King Arthur. Remember this is before Lancelot had been "invented", and Mordred fills the adulterous role with Guinevere that Lancelot will eventually play. While Arthur is away fighting the Roman general Lucius in France, Mordred marries Guinevere and claims Arthur's throne for himself. Arthur returns from France to try to regain the throne, and fights a series of battles against Mordred. The last of these battles is Camlann.
Medieval French authors are the first to introduce incest. In the Vulgate cycle two different stories of the incestuous conception of Mordred appear. In one, Arthur and his sister, the beautiful wife of King Lot of Orkney, commit incest unknowingly, discovering afterwards that they are brother and sister. In another she is deceived by Arthur into thinking that he is her husband, Lot. Either way, the result of this union is Mordred.
Thomas Malory's is the best known version of the story. Malory tells of Arthur heeding Merlin's prophecy that the child who would bring about the fall of Camelot would be born on May-Day. Therefore Arthur rounds up all children born on that day in his kingdom, puts them on a ship and casts them loose. The ship hits the rocks, and all the children except one are drowned. The one who survives is Mordred, who is fostered by his rescuers until the age of 14. Throughout Malory Mordred appears now and again but does not figure importantly until towards the end. Mordred attempts to take over the kingdom while Arthur is away at the Siege of Benwick. This is then leads to the final fight between Arthur and his treacherous son, and Arthur's fatal would and the death of Mordred.