Lady of Avalon, by Marion Zimmer Bradley

Lady of Avalon, the eagerly awaited link between Forests of Avalon and The Mists of Avalon, spans the creation of Avalon itself, right up to the birth of King Arthur.

This enchanting novel of the Holy Isle is told through the successive lives of its three most powerful priestesses: Caillean; Dierna; and finally Viviane, high priestess of Avalon and Lady of the Lake who safeguards the Grail.