Mordred

Mordred was a nemesis of King Arthur and the son of the sorceress Morgaine or Morgan.

Biography
According to one account, Mordred's sorcerous mother was called Morgan and was the human half-sister of King Arthur. His birth was arranged by the Master in the knowledge that he would one day bring about Arthur's death. Arthur found out about his birth in Lothian, the domain of Morgan's legitimate husband Lot of Lothian, and sent the Merlin there to kill the baby, little knowing that "the Merlin" and the Master were one and the same. The Master instead spirited the child away to be hidden and raised in a place of safety until his time came. (PROSE: The Creation of Camelot) Another account suggested Mordred's mother Morgaine was not related to Arthur. (TV: Battlefield) According to one source, the Arthurian legend was a reflection of a conflict between the Guardian of Magic and the Guardian of Might, whose Earthly incarnations were Morgan/Morgaine and Arthur; as such, it replayed endlessly with shifting details. (PROSE: Legends of Camelot)

A formidable knight and a skilled swordsman, Mordred still depended on his mother's magical abilities. He and the knight Ancelyn both displayed an ability unusual to humans native to the Doctor's world: each was able to recognise the Seventh Doctor as Merlin on sight, despite never having met that incarnation of the Doctor before. (TV: Battlefield)

Mordred stabbed King Arthur with his blade in Camalan. (PROSE: The Door We Forgot)

At some point after the Battle of Camlann, in which King Arthur was killed, Mordred crossed from his own world to the Doctor's. Once he had crossed dimensions, he summoned his mother there as well, using a magical ritual and a sword that he called "brother to Excalibur."

Along with Morgaine, he was put into military custody by UNIT following her surrender. (TV: Battlefield)

Behind the scenes

 * Mordred is an important character in Arthurian legend: son of King Arthur and Morgause, who were half-siblings. Mordred is Arthur's deadliest foe. Mordred's forces faced Arthur's at the apocalyptic Battle of Camlann, in which Mordred died having dealt a fatal blow to Arthur, who died soon after. In Doctor Who, Morgaine mentions a romantic encounter with Arthur, but the identity of Mordred's father is never stated.