Mordred was the son of King Arthur and Morgause, and a knight of the Round Table.  Eventually, he betrayed Arthur and destroyed Camelot. He died fighting Arthur, but mortally wounded his father in the process.  Arthur was taken to Avalon by The Three.
Real World Background
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 Historia Regum Britanniae developed the more familiar image of Mordred as a traitorous nephew to Arthur, while the French Vulgate Cycle added the notion of the incest.
According to Sir Thomas Malory, Mordred was begotten when Morgause visited Arthur's court early in his reign, on an embassy from her husband King Lot of Lothian and Orkney, one of Arthur's greatest enemies. Arthur was taken with Morgause and slept with her, unwittingly committing incest. Merlin afterwards told Arthur what he had done and how 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 du Merlin, Malory's source, offers a slightly different account; instead of attempting a Herod-like massacre on the babies, 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 fourteen, but provides no details. The Vulgate Cycle states that Mordred began his career as a good knight, until one day he and Sir Lancelot met a hermit who told them that both would bring Arthur's kingdom down; he started with Mordred, telling 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 (before the hermit could turn to Lancelot and predict his role in the fall of Camelot). Afterwards, his mood darkened.
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 been having an affair with Morgause) and Sir Dinadan (who had unhorsed Mordred in a joust once), 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 (she escaped, however, and took refuge in the Tower of London). Learning of his son's treachery, Arthur returned to Britain to fight him.
After a few inconclusive battles, Arthur and Mordred prepared to face each other on Salisbury Plain. 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 almost everyone on both sides was slain; only Arthur, Mordred, and two of Arthur's knights, Sir Lucan and Sir Bedivere, were left alive. Arthur resolved to finish his son forever and ran him through with a spear; Mordred, in his death-throes, smote Arthur with his sword, mortally wounding him.)
- Mordred at Wikipedia, the Free Encyclopedia