ARTHUR'S SISTER'S STORY

THE MISTS OF AVALON By Marion Zimmer Bradley. 876 pp. New York: Alfred A. Knopf. $16.95.

OF the various great matters of Western literature - the story of Troy, the legend of Charlemagne, the tales of Araby - none has more profoundly captured the imagination of English civilization than the saga of its own imperial dream, the romance of King Arthur and the Round Table. Two national epics, Spenser's ''Faerie Queene'' and Tennyson's ''Idylls of the King'' - one Elizabethan and one Victorian, both dedicated to reigning queens - recreated the enchantments of Arthur to anatomize the British Empire in different ages. This legend has always proved capacious enough for less august modes too: spoofery from Chaucer to Mark Twain, love poems, operas and in our time the vast multivolume novels of Mary Stewart and T.H. White, whose ''Once and Future King'' even made it to Broadway as ''Camelot.''

The story of Arthur traditionally begins as the story of male lust. Thomas Berger rendered this origin of the saga in the opening sentences of his hilarious and exquisite ''Arthur Rex'': ''Now Uther Pendragon, King of All Britain, conceived an inordinate passion for the fair Ygraine, duchess of Cornwall, and having otherwise no access to her, he proceeded to wage war upon her husband, Gorlois, the duke.'' Five centuries ago Thomas Malory started his ''Morte d'Arthur'' with two effectively prurient paragraphs on the same desire.

In ''The Mists of Avalon,'' Marion Zimmer Bradley's monumental reimagining of the Arthurian legends, the story begins differently, in the slow stages of female desire and of moral, even mythic, choice. Stepping into this world through the Avalon mists, we see the saga from an entirely untraditional perspective: not Arthur's, not Lancelot's, not Merlin's. We see the creation of Camelot from the vantage point of its principal women - Viviane, Gwynyfar, Morgaine and Igraine. This, the untold Arthurian story, is no less tragic, but it has gained a mythic coherence; reading it is a deeply moving and at times uncanny experience.

In Mrs. Bradley's novel Viviane is the Lady of the Lake, High Priestess of Avalon and sister of the Lady Igraine. In a vision granted by the Great Goddess, Viviane has foreseen a Britain united in peace under a high king who will remain true to Avalon and the old religion of pagan Goddess worship while tolerating the new religion of the male Christ that is now winning its way across the land. Viviane accordingly chooses her sister, Igraine, to give birth to this future king, Arthur. She also chooses and trains Morgaine, Igraine's daughter and therefore Arthur's half-sister, to succeed her as priestess of the mysteries of Avalon. However, Viviane's plan to insure a doubly royal heir for Arthur goes awry: She selects Morgaine as the priestess-virgin to be deflowered in the primitive ritual Arthur must carry out to become king. Horrified to learn that this incestuous union with her half-brother has made her pregnant, Morgaine leaves Avalon, abandoning her duty as High Priestess and sowing the seed of future tragedy. Thus Mrs. Bradley gives us a plot behind the plot of the Arthurian story as we have known it.

It is the last of the four heroines, the Christian queen, Gwynyfar, who pushes forward the inexorable destruction of Camelot. Driven to a maddened piety by her continued barrenness and by guilt over her unconsummated love for Lancelot, Gwynyfar begs Arthur to break his oath to Avalon: If he flies the banner of the Cross and Virgin in place of that of the Dragon, if he consecrates the chalice of the Goddess's Holy Regalia by using it in the Christian mass, perhaps Gwynyfar will be able to conceive an heir.