Mordred

Mordred was the son of Morgaine, a powerful sorceress in an alternate Earth where King Arthur actually existed.

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.