Morgause

Morgause, a skilled warrior and a powerful sorceress, is Morgana's half sister. (The Sins of the Father)

Early life
Morgause is the half-sister of Lady Morgana. She was born into one of the houses of Camelot, the house of Gorlois.

For unknown reasons, Gaius was asked to smuggle Morgause out of Camelot after she was born and give her to the high priestesses of the Old Religion. Gaius did so, and swore an oath to never reveal his actions to anyone. Uther was led to believe that Morgause had died.

Morgause studied magic for many years, eventually becoming a very powerful sorceress.

First attack on Camelot
Many years after her birth, Morgause returned to Camelot, now a grown woman. Disguised as a knight, she killed five guards to get into the castle, where she revealed herself to the court and challenged Arthur Pendragon to a duel. Despite the fact that he did not wish to fight a woman, Arthur agreed. The next day, Morgause defeated Arthur in their duel, but spared him on the condition that he visit her and accept her next challenge. She also introduced herself to her half-sister, Morgana, though she kept their relations a secret. She also gave her a bracelet that belonged to their family that had magical healing powers that would help ease her nightmares. Gaius found the bracelet and informed Uther that the two young women maybe related. As she was leaving Camelot, she told Arthur that she knew something about his mother, giving Arthur incentive to come see her. Despite the fact that his father tried to prevent him from leaving, Arthur went to meet her, accompanied by his manservant and friend Merlin. When they arrived at her domain, she told Arthur to rest his head on a chopping block so she could kill him. He did so, and Morgause told him the whole thing had been a test of his character. As his reward, Morgause used her magic to summon Ygraine's apparition, which told Arthur the secret of his birth: he had been concieved by magic because Uther wanted an heir and she had to die as a result.

Enraged at his father for his selfishness, Arthur returned to Camelot and attempted to kill Uther. But just as he was about to deal the fatal blow, Merlin burst into the hall and convinced him that Morgause had tricked him. Morgause was upset that her plans had failed. Her manipulations, meanwhile, have deepened Arthur's hatred of magic and therefore Merlin will probably have an even harder time when he has to tell Arthur he's a warlock. (The Sins of the Father)

Second attack and taking Morgana
Morgause later awoke the Knights of Medhir to kill Uther. She also approached Morgana, and got her on her side. She then placed a spell upon Morgana, binding her life to a sleeping spell that she placed on Camelot. The spell put everyone in Camelot to sleep, with the exception of Morgana, Arthur and Merlin (who were away when she cast the spell). Merlin, after visiting The Great Dragon, learned of how to stop the Knights, and reluctantly poisoned Morgana with hemlock. Morgause, upon hearing her sister's telepathic screams, confronted Merlin and demanded he tell her what he used to poison her sister so she could heal her. Merlin refused to do so unless she killed the Knights. Morgause chose her sister over her chance to kill Uther, and used her magic to kill the knights. Merlin showed her the hemlock, but Arthur and the Knights of Camelot soon burst into the room. Surrounded, Morgause used her magic to take Morgana away from Camelot and saved her life. The sisters were united to start planning the attack of Camelot. (The Fires of Idirsholas)

Return to Camelot
At the start of the season with the aid of her half sister she casts a powerful enchantment on Uthur useing a mandrake root and his tears. The spell causes Uthur to hallucinate seeing people who have died because of his choices and actions before and durring the purge. These include his wife frightened and bleeding, begging Uthur not to harm her and an unkown drowned boy; who is revealed by Gaius to be one of the many children Uther had killed simply because they may have inherited magic from their parents. The spell gives the impression to the people of Camelot that their king may be losing his mind and possibly turn on him as a result. She has also been working with King Cenred to move against Camelot.

Prior to the attack she and Morgana capture Merlin who had been spying on them and tried to warn Arthur. After binding Merling in magical chains she interrogated him; curious to know why a "servant" as lowly as Merlin would go out of his way to protect Arthur and Camelot. Merlin was evasive and did not give a definative answer especially when Morgause senses he is hiding something. She seemed prepared to let Merlin live thinking him no real threat but then tightens the chain informing him she was well aware of his attempt to poison Morgana which she did not take kindly to. She then leaves to attack Camelot leaving Merlin at the mercy of the Serkets only for him to be rescued by Kilgharrah.

Personality
Morgause is highly intelligent and manipulative. She took advantage of Arthur's interest in his mother to try to turn him against Uther. This almost resulted in Arthur killing his father. She also used Morgana's hatred of Uther to persuade her to join her. She is consumed with hatred and rage as a result of sorcerers persecution by Uther whom she will stop at nothing to destroy.

Morgause is largely callous and cold towards others, viewing most of them as tools to be used and disposed of. However she does have a softer side. When she found out Morgana was dying, she abandoned her attempt on Uther's life in order to save her half-sister. She attempted to kill Merlin for poisoning Morgana and also acted as a confidant to her sister. Together they plotted to bring down Uther. She also knew King Cenred, who was possibly attracted to her, since he was eager to impress her and commented on her courage. Morgause was a confident woman, striding straight into Camelot without attempting to be subtle about it and killing five guards on the way before challenging Arthur to a duel. She also seemed fairly confident that she could defeat him (which she did).

Morgause is intrigued by Merlin but is currently unaware of his magical powers, thinking he is just a servant. However she began to suspect that he was hiding something.

Morgause claimed to have known Queen Ygraine of Camelot very well, saying she was a woman of honor. Given this statement, Morgause is at least 10 years older than Arthur. It is also interesting to note, that Morgause and Arthur have a physical resemblance, which could lead one to believe that they are related.

Abilities
Morgause is a very powerful sorceress able to summon an apparition of Arthur's mother and later reawaken the Knights of Medhir. She is also an exceptionally skilled swordswoman, being able to kill five Camelot guards with ease and was the only known person besides Lancelot and possibly Morgana who has managed to defeat Arthur in combat.

Legends
Morgause is the sister or half-sister of King Arthur in the Arthurian legend. In her earliest appearance she is Arthur's full sister by Uther Pendragon and Ygraine; and is the mother of the heroic Gawain and the villainous Mordred. In later works she is the daughter of Ygraine by her first husband, Gorlois, and she becomes pregnant with Mordred after sleeping with Arthur while they were unaware of their relation. Her husband is King Lot, an enemy of Arthur in the rebellions following Arthur's coronation.

Her other siblings include sisters Elaine and Morgan le Fay, although many works seem to combine Morgause and Morgan Le Fay, so they may well have been the same person.

Morgause at wikipedia