Mordred

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

Biography
A formidable knight and a skilled swordsman, Mordred still depended on his mother's magical abilities. He killed the fiancée of Arthur's knight, Ancelyn, after which the latter swore revenge. He and Ancelyn both displayed an ability unusual to humans native to the Doctor's world: each was able to recognise the Doctor as Merlin on sight, despite never having met that incarnation of the Doctor before.

At some point after the Battle of Camlann, in which King Arthur was killed, Mordred crossed from his own world to the Doctor's by unknown means. 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. (DW: Battlefield)

Behind the scenes

 * Mordred is an important character in Arthurian legend: son of King Arthur and Morgaine (who were half-siblings), Mordred is Arthur's deadliest foe. Mordred's forces faced Arthur's at the apocalyptic Battle of Camlann, in which Arthur was mortally wounded. In Doctor Who, Morgaine mentions a romantic encounter with Arthur, but the identity of Mordred's father is never stated.