Mordred, also called "Sir Mordred" is one of the most famous antagonists of Arthurian legend and the one directly responsible for the great King Arthur's death - he is often envisioned as either a rival or archenemy of Arthur depending on the legend.
In Dante's Divine Comedy, Mordred fell into Caina, the first cycle of the ninth realm of the Inferno, Cocytus, for he betrayed King Arthur who is his uncle and was responsible for Arthur's death.
Mordred is first mentioned in Arthurian writings, under the name "Medraut", in the Annales Cambriae, in the entry for the year 539, where Arthur and Medraut were mentioned as both slain at the Battle of Camlann. The fragmentary Welsh writings about Arthur suggest that Medraut was imagined as a rival to Arthur rather than a traitorous follower; one of the Triads says that Medraut raided Arthur's hall once, and Arthur carried out an equally violent raid on Medraut's hall in retaliation. Geoffrey of Monmouth's History of the Kings of Britain developed the more familiar image of Mordred as a traitorous nephew to Arthur, while the French Vulgate Cycle added the notion of the incestuous birth. In earlier stories, King Arthur is Morded's uncle and King Lot is his father.
According to Sir Thomas Malory, Mordred was begotten when Morgause, Arthur's half-sister, visited his court early in his reign, on an embassy from her husband King Lot of Lothian and Orkney, one of Arthur's greatest and most dangerous enemies. Arthur was taken with Morgause, and unknown to himself, committed incest. Another variation says that Arthur was put under a spell, to do this, as planned by the evil sorceress that the incestuous seduction would produce an illegitimate son to control or destroy his father. Merlin afterwards told Arthur what he had done and warned that Mordred would grow up to kill him. Alarmed, Arthur sent for all the infants born on May Day (the day that Mordred would be born on) and sent them in a boat out to sea; the boat hit a rock and sank, drowning all the babies except for Mordred, who was rescued from the wreck. (The French Suite de Merlin, Malory's source, offers a slightly different account, wherein Arthur had decided to send them to a remote corner of his kingdom where they could be watched over in isolation; the boat that sank with Mordred on board was a ship bringing the babies from Scotland to Arthur's court. It is uncertain whether Malory's account is a deliberate change from the story in the Suite, or a misleading abbreviation.)
Malory says that Mordred came to Arthur's court when he was 14 years old, but provides no details. The Vulgate Cycle states that Mordred began his career as a good knight, until he and Sir Lancelot met a hermit who told the young man of his true parentage as Arthur's incest-born son, and how he would destroy everything his father had built. Mordred lost his temper and killed the hermit at once, but was morose ever after.
Mordred murdered or helped murder two of Arthur's knights, Sir Lamorak (as part of a family feud; Lamorak's father, King Pellinore, had slain King Lot in battle, and Lamorak had an affair with Morgause) and Sir Dinadan (who had unhorsed Mordred in a joust), in partnership with his half-brother Sir Agravain. Both became aware of Lancelot and Guinevere's affair and decided to expose it, resulting in the civil war between Arthur and Lancelot that brought about the fall of Camelot. When Arthur pursued the exiled Lancelot to Gaul, he placed Mordred in charge of Britain in his absence; Mordred promptly usurped the throne and even tried to marry Guinevere (who took refuge in the Tower of London). Learning of his son/nephew's treachery, Arthur returned to Britain to fight him.
After a few inconclusive battles, Arthur and Mordred prepared to face each other at Camlann. Arthur experienced warning dreams the night before urging him not to fight his son that day, and so called for a parley. During the parley, however, an adder bit one of the knights, who, without thinking, drew his sword to kill it; both armies mistook it as an act of treachery, and the battle began. It lasted until nightfall, when virtually everyone on both sides was slain just as Arthur's dream prophesied; only Arthur, Mordred, and two of Arthur's knights, Sir Lucan and Sir Bedivere, were left alive (Sir Lucan would ultimately succumb to his own grievous injury). Arthur, furious, pierced Mordred with a spear; and Mordred, in his death-throes, smote Arthur with his sword, mortally wounding him.
In the Alliterative Morte Arthur, the story is much the same as in Malory, except in the single combat at the end, wherein Arthur and Mordred fight with swords, and Arthur cuts off Mordred's sword-hand, before killing him and dying thereafter.
Most later versions of the character follow either Malory, or the Alliterative Morte, as in T.H. White's Once and Future King, which retells Malory almost verbatim.