My favourite
villain in
the whole wide world.
She was the daughter of Gerlois, the Duke of Cornwall, and his wife, Igraine. When she was but a child Uther Pendragon, king of England, fell in love with her mother, as she and her husband came to his coronation to swear fealty. Uther tried to persuade Igraine to sleep with him, but she refused. In his lust Uther Pendragon lead the entire english army to Cornwall, and besieged Tintagel, Gerlois's Castle. When Gerlois left the castle in order to fight, Uther ordered Merlin to cast a spell on him that will change his appearance so that he'd look just like Gerlois. Merlin agreed but had two conditions: that Gerlois will not be killed, and that the child of Uther and Igraine will be given to his custody. In this disguise Uther entered Tintagel and slept with Igraine. At the same night, against his promise to Merlin Uther ordered Gerlois killed. From Igraine's and Uther's consumation Arthur was born, and as promised (Uther could not back out now) he was given to Merlin.
Morgana was just a small child at the time, and therefore no-one noticed her, but she was smart and resourceful, and had some little rudimentary magical talent, she also was very much attached to her father. She saw through Uther's disguise and swore eternal enmity to him and his son.
She turns to Merlin and asks him to teach her magic, he is impressed by her beautiful and seemingly-innocent appearance, as well as by her great intellect, and agrees to teach her. After she learns all that she can from him she turns to other teachers and learns darker, more dangerous arts than the one Merlin taught her. Appearantly at that time she manages to transform from human to faerie.
Using the very spell Merlin cast on Uther to get her mother, she managed to sneak into her half-brother's chamber and sleep with him. The child born from this union she called Mordred. She raises this son to hate and despise his father, and to plot, along with her, his downfall.
Morgana managed to steal excalibour from the sleeping Arthur and tried to destory the sword, but Arthur manages to take it back before she executes her plan. However, Morgana does manage to lose the scabbard of the sword, that protects the magical blade from rust and the injuries of time, and the wielder of the blade from all injury, forever.
Morgana sent Mordred, as he reaches manhood, to his father, and the father appoints him regent to the throne, along with the queen, Guinevere, as he himself goes to war in Gaul.
Morgana is the one who first finds out about Guinevere and Lancelot's adultury, and reveals this betreyal to Arthur, giving him a cup with a magic drought, which when he drinks reveals to him a vision of the two while in the act. Mordred, by Morgana's command, persuades Arthur that he has no other choice but executing the queen for high-treason.
When Guinevere is saved from the stake by her lover, Mordred blames Arthur of not really intending to execute the queen, and for planning, along with Lancelot her rescue. Mordred blaims the king of percieving himself above the laws, and declares a rebellion.
The war that ensues culminates in a battle along the river Camel in Cornwall (according to several versions, the battle takes place in Salisbury but I prefare the cornish version, as it is more symbolic, where everything started, that's also where it ends). In this battle Mordred and Arthur both die at the other's sword.
This is where Morgana's role changes: up till now she is her brother's archnemesis, but here she is depicted as the principal of the Three Queens, who come down the river on a boat to take Arthur's body to Avalon. Where he can be preserved until the time England most needs him.
In many versions of the legend Morgana is described as Queen of Avalon, and although she is not always benevolent (in "Sir Gawain and the Green Knight" she tries to seduce Gawain, in "Orlando Innamorato" she is a witch-like spirit), She does have good qualities, thus she accepts Ogier to avalon in his old age (in "Ogier the Dane"), and in "Orlando Furioso" she gives from her treasures to those she likes.