Morgause (played by Emilia Fox) is Morgana's half-sister (according to Gaius she is Gorlois' daughter) who was smuggled out of Camelot by Gaius after her birth, for reasons unknown, to be raised by the priestesses of the Old Religion, learning powerful magic during her time with them. Returning to Camelot many years later, she challenged and defeated Arthur in a duel, but promised to spare his life if he came to meet her later and accept her challenge. Having passed her test, Arthur willingly placing his head on a block while Morgause held an axe as though about to behead him, and proven his honour. Morgause then summoned an apparition of what appeared to be Arthur's mother, which revealed that Uther had used magic to ensure Arthur's birth. Morgause had apparently hoped that this revelation would drive Arthur to kill Uther. Her precise motives for desiring Uther's death are unknown at this time, beyond the obvious detail of Uther's hatred of magic.

Merlin was able to halt their fight by claiming that the apparation was nothing but a deception, despite the fact that Uther's death would have made his own life easier and the lie has only increased Arthur's own hatred of magic. Morgause seems furious that her plan had failed. Morgause, while in Camelot, seemed to immediately recognize Morgana and gave her the bracelet bearing Gorlois' house's crest (which is how Gaius identified her). After learning that Morgana suffered night terrors, Morgause visited her while she slept and cast a spell to help her sleep better.

In her second appearance, Morgause intends to revive the deadly Knights of Medhir, and use them in her plans to destroy Uther. This plan includes casting a spell to put the whole of Camelot to sleep, using Morgana as its 'host', but she is forced to abandon this plan when Merlin- based on instructions from the Great Dragon- is forced to poison Morgana in order to break the spell, Merlin providing Morgause with the poison he used on Morgana so that she could be cured in exchange for Morgause abandoning the attack and ending the spell.

Morgause and Morgana returned in the third season, now allied together to destroy Uther. Although Morgana attempted to drive Uther insane with a mandrake spell that caused him to experience visions of those whose deaths he had caused, her treachery was uncovered by Merlin, only for him to be captured and interrogated by Morgause. When Morgause was unable to get any information out of Merlin beyond him informing her of his belief in a just Camelot under Arthur's reign, she bound him in enchanted chains and left him to die, only for Merlin to later escape with the aid of the Great Dragon. During the subsequent siege of Camelot, Morgause expressed confidence in Morgana's ability to wield her magic to raise the dead inside the castle, but was left shaken when the plan failed.