Morgaine

Morgaine, also known as The Sunkiller, Dominator of the Thirteen Worlds, and Battle Queen of the S'rax, was a powerful sorceress from another dimension, who did battle with King Arthur and Merlin.

In Arthur's World
Morgaine was an enemy of King Arthur and Merlin. Her son Mordred, with the Knight Commander, led Morgaine's forces: knights with advanced weaponry and the capability to travel between dimensions.

Morgaine sealed the Doctor of her universe, called Merlin, into the ice caves for all eternity but he escaped. She waited twelve centuries to face him again.

In our universe
Mordred created a meeting point after crossing to Carbury, England, allowing Morgaine to follow him across the realities. They then set out to prevent Ancelyn and the Seventh Doctor (Merlin in her universe) from possessing Excalibur. She unleashed the Destroyer to help, but it was defeated. Morgaine made a final attempt at victory by unleashing a nuclear weapon from a nearby missle convoy, but the Doctor convinced her that this was a dishonourable form of warfare and she surrendered. Morgaine and Mordred were taken prisoner by UNIT. (DW: Battlefield)

Personality
Morgaine had a strong sense of honour. She became enraged when, by reading a tombstone, she discovered her son, Mordred, had misled them and they had fought on the enemy's soil without proper respect for the dead. She had incredible powers, being able to control people and cure blindness. (DW: Battlefield)

Behind the scenes

 * In the real world, she is sometimes known as Morgan le Fay, Morgain, or Morgana. She was a sorceress of Arthurian legend often described as the half-sister of King Arthur. In the legends it was Morgan's sister Morgause who was the mother of Mordred by an incestuous affair with Arthur, but Morgan and Morgause are often conflated in later retellings.
 * Morgaine actress, Jean Marsh, portrayed Sara Kingdom; short lived companion to the First Doctor.