The Mists of Avalon by Marion Zimmer Bradley

Here is the tragic tale of the rise and fall of Camelot - but seen through the eyes of Camelot's women: The devout Gwenhwyfar, Arthur's Queen; Vivane, High priestess of Avalon and the Lady of the Lake; above all, Morgaine, possessor of the sight, the wise, the wise-woman fated to bring ruin on them all...