In a dimension that seems to be parallel to ours, the Earth is populated by people of legend - specifically, the tale of the Knights of the Round Table. The knights that exist in this dimension wear medieval clothing and fight with swords, but they are also technologically advanced - they utilize long-barrelled energy weapons and grenades, as well. They can also grow advanced bioengineered spaceships which are powered magically. Their native tongue seems to be much like Welsh, although it may only exist in naming tradition. To them, our world is "Avallion", and a convocation - a ritual which bridges a point between the two realities - allows them to travel to it. This ritual may only be available to those with magical powers; the knights seem to arrive on Earth by travelling (unprotected?) through space.
Leader of these people is Morgaine of the Fay, otherwise known as Morgaine the Sun-Killer, Dominator of the Thirteen Worlds and Battle Queen of the S'Rax. Morgaine is an extremely powerful sorceress: she can send energy blasts from her fingers which can disable machines and turn bodies to dust; she can communicate telepathically with her knights; she can catch bullets with her bare hands and crumble them to dust; she can absorb information directly from a person's brain, which kills them in the process; and she can exert a psychic control over people, influencing them to do what she wants. She possesses an opaque white ball which reveals to her scenes from other places. It can either fly on its own, or it does so through the power of Morgaine's mind. She may also be able to establish interstitial vorteces, which look like a rotating column of lights, for travelling purposes.
Morgaine's deep sense of honor is matched by her deep determination to conquer all that she can. She can be cold, cruel, and calculating, and she exudes an icy ferocity that makes it clear to everyone that she is not to be crossed. Morgaine is a woman who would casually kill anyone with merely the wave of a hand, and would even sacrifice her son Mordred if she had to. She has waited twelve centuries to face Merlin, who she bound into ice-caves for all eternity, in battle. She has been waging war against Arthur for a very long time, and yearns to fight him in single combat.
But there is another side to Morgaine which rarely reveals itself. She may be callous at times, but she genuinely cares for her son, and once she harbored deep feelings for Arthur. She is not against using her power for good - she repaid a blind woman for four drinks by restoring her sight. And Morgaine is not without her weaknesses. Her magic is powerful, but it cannot breach a chalk circle, particularly when its occupants are protected by the sword Excalibur.
Morgaine is a tall woman, clothed in copper and bronze. She wears a breastplate, chain mail, a cape and a crown, befitting her status as Queen. Her hair is a bright red, her eyes icy blue, and her fingernails are more like talons.
Her mission, once her war against Arthur's forces spills over into Avallion, is to recover Excalibur and use it to awaken Arthur. Morgaine brings with her insurance in the form of the Destroyer, who, if loosed, can destroy the world. She fears the Destroyer's power, but not enough to keep him chained forever. What Morgaine does not realize is that Arthur died a long time ago, and his body has turned to dust. When the Destroyer is defeated, Morgaine tries to unleash a nuclear missile, but the Doctor persuades her to stop by telling her of Arthur's death. When Morgaine realizes that her battle has lost its meaning, she aborts the missile launch. The Doctor instructs UNIT to put her, and her son Mordred, in jail.
