Morgan le Fay is a sorceress, mostly associated with the Arthurian legend. Her relationship to King Arthur and her motivations change drastically depending on the story, but pretty much all of them portray her as being a nasty sort. Some of the tales have her being a mystical being while others refer to her as a human with magic powers. Not only is she mentioned in early Brittanniae tales, but she is also exists in French and Italian folktales. Fay is an Old French word for Faerie.
Actress Helen Mirren portrayed her in the 1981 movie, Excalibur.