Morgause was very confident in her own abilities, passing straight into Camelot and killing five guards on the way before challenging Arthur to a duel. She also seemed confident that she could defeat him, which she did. The sisters were united to start planning the attack of Camelot (The Fires of Idirsholas). Morgause despises Arthur, not realizing until Uthr has died that he is Uthr’s son, but thinking he is only Uthr’s bastard-born nephew. Actually I do recommend people read Alchemy (Part 1), it helps set up part 2. When Morgause first appears in the series, we do not know (nor does Gaius) that Uther is Morgana’s father.
When she found out Morgana was dying, she abandoned her attempt on Uther's life in order to save her half-sister.
She also introduced herself to her half-sister, Morgana, though she kept their relationship a secret. The The Once and Future King quotes below are all either spoken by Queen Morgause or refer to Queen Morgause. Soon, Morgause … She later attempted to kill Merlin for poisoning Morgana and also acted as a confidante to her sister. She was enraged when Merlin thwarted her and Morgana's plan, though she did not know that Merlin was the person that prevented the fall of Camelot (The Tears of Uther Pendragon).
Morgause was so consumed with hatred and rage as a result of the persecution of sorcerers by Uther that she would stop at almost nothing to destroy him completely.
If Gaius believed Morgause had the same two parents as Morgana he would simply refer to her as a sister. The pale boy said "But-" he was interrupted by a loud voice "Soldiers!" Merlin then showed her the hemlock, but Arthur and the Knights of Camelot soon burst into the room. Morgause was the half-sister of Morgana Pendragon; it is currently unknown if she had any other siblings, however this is more on the unlikely side.
Merlin refused to do so unless she killed the Knights. Enraged at his father for his presumed selfishness and without knowing all the details, Arthur returned to Camelot and attempted to kill Uther. Mostly Morgana or Arthur POV.
She told Cenred to get the Cup for her.
He did so, and Morgause told him the whole thing had been a test of his character. Morgause studied magic for many years, eventually becoming a very powerful sorceress, acting to take her revenge on King Uther Pendragon.
Her manipulative and carefully scheming ways were shown by her use of Arthur's curiosity about his mother to try to turn him against Uther.
For some unknown reason, Gaius was asked to smuggle the infant Morgause out of Camelot and give her to the High Priestesses of the Old Religion. Her mother was Morgause, and her father was Keros, the dominant pair of the Young Ones at the time. Morgause manipulated Guinevere and Sir Leon into leading them to Arthur who had escaped Camelot along with Merlin, Elyan, Gwaine, and Gaius.Lancelot and Percivalantervened allowing the group to escape. Morgause called out, it seemed like Arthur and Merlin were having a heated conversation, "It's just Merlin" Arthur said as Morgause and Morgana joined them "Hi Morgause, I'm Myrddin Emrys." Morgause was later involved in a brief ceremony where Morgana was crowned Queen of Camelot, after her immortal warriors defeated Camelot's army. With Katherine Langford, Devon Terrell, Gustaf Skarsgård, Daniel Sharman. She also gave her a bracelet with magical healing powers to relieve her of her nightmares, saying it belonged to her mother, thus implying that it was also Morgana's mother. Morgause(VAF005) was First Seen on January 24, 1994, in a wild group called the Avatar. Morgause also arrogantly informed Merlin that she wouldn't see him again as she prepared to kill him and was caught off guard by the arrival of Gaius which resulted in her defeat.
As his reward, Morgause used her magic to summon either Ygraine's spirit or the illusion of Ygraine, who told Arthur the secret of his birth; he had been conceived by magic because Uther had wanted an heir and as a result she had died. Morgause was later seen working with King Cenred to move against Camelot and watched the siege of Camelot with him.
A later search through the rubble found no trace of Morgana or Morgause leaving their fate unknown (The Coming of Arthur). Cenred's men succeeded in collecting the Cup and Morgause made Cenred's army immortal. Morgause later met up with Morgana in a crowded market disguised as a poor old woman. As she was leaving Camelot, Morgause told Arthur that she knew something about his mother, giving Arthur incentive to come see her.
Mordred is the offspring of Arthur's inadvertent incest with Morgause, the king's estranged half-sister. Morgause’s hatred for Arthur causes her later to attack him sexually. Holly(VYF008) was born on December 11, 1996, in the Young Ones mob.
She was a skilled warrior, a powerful sorceress and a High Priestess of the Old Religion. Morgause was upset that her plans had failed while her deceit further cemented Arthur's mistrust of magic (The Sins of the Father). Merlin then defeated Morgause by smashing her against a pillar with his magic, incapacitating her.