Which sorceress was King Arthur's half-sister?

The sorceress who was King Arthur’s half-sister is Morgan le Fay.

Morgan le Fay is one of Arthurian legend’s most powerful enchantresses. In the best-known medieval romance tradition, she is the daughter of Igraine and Gorlois, making her Arthur’s maternal half-sister because Arthur is Igraine’s son by Uther Pendragon. Her family relationships vary among authors, but this half-sister identity became the dominant version.

Morgan’s earliest major appearance is in Geoffrey of Monmouth’s *Vita Merlini*, written around 1150. There she is associated with Avalon and appears chiefly as a magical healer. Later French romances transform her into a more morally ambiguous figure: sometimes Arthur’s ally, sometimes an enemy of Guinevere, and often a schemer against the king’s court.

Morgause is a common mix-up because she is also Arthur’s half-sister in many versions. She is usually portrayed as the mother of Gawain and Mordred, whereas Morgan le Fay is the character most specifically identified as an enchantress or sorceress.