"Mists of Avalon" by Marion Zimmer Bradley.
It is a 1983 novel by Marion Zimmer Bradley, in which she relates the Arthurian legends from the perspective of the female characters. The book follows the trajectory of Morgaine (often called Morgan le Fay in other works), a priestess fighting to save her matriarchal Celtic culture in a country where patriarchal Christianity threatens to destroy the pagan way of life. In stark contrast to most other retellings of the Arthurian tales, which consistently cast Morgan le Fay as a distant, one-dimensional evil sorceress, with little or no explanation given for her antagonism to the Round Table, Morgaine is presented as a woman with unique gifts and responsibilities at a time of enormous political and spiritual upheaval who is called upon to defend her indigenous matriarchal heritage against impossible odds.
There was also a TV miniseries made: