Mordred

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

Profile
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. Mordred also survived the Battle of Camlann. Crossing from his own world to the Doctor's by unknown means, he summoned his mother there as well using a magical ritual.

Mordred immediately recognised the Doctor as Merlin, though neither man had seen that particular incarnation of the Doctor. Ancelyn was ultimately unable to defeat Mordred in single combat. With Morgaine 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.