Mordred

Mordred was the son of Morgaine, a powerful sorceress in an alternate Earth where King Arthur really existed. Mordred is a fierce and formidable knight and a skilled swordsman, though he also depends on his mother's magical abilities.

Like Ancelyn, Mordred recognised the Doctor as Merlin on sight, though neither man had seen that particular incarnation of the Doctor.

At some point before the events of DW: Battlefield, Mordred killed Ancelyn's fiancee'; Ancelyn vowed revenge, but was ultimately unable to defeat Mordred.

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 final, apocalyptic Battle of Camlann, where Arthur was mortally wounded.