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 the enemy of King Arthur and Merlin, her son Mordred and the Knight Commander leading knights with advanced weaponry and the capability to travel between dimensions against them. Morgaine eventually sealed Merlin, the Doctor of her universe, into the ice caves for all eternity but he escaped and she waited twelve centuries to face him again. (TV: Battlefield)

In our universe
Although she should never have appeared on Earth at any time, Morgaine was attracted there in 1997 by the Mandragora Helix. (COMIC: Distractions)

After crossing to Carbury, England, Mordred created a meeting point, 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 missile 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. (TV: Battlefield)

Personality
Morgaine was ruthless, willing to sacrifice almost anything, including her son Mordred, to achieve her ends. Despite this, she had a strong sense of honour. She became enraged when, by reading a tombstone, she discovered that 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. (TV: 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.
 * She was played by Jenny Agutter in The Twilight Zone.