Marion Zimmer Bradley's Ravens of Avalon: Avalon Book 6

Marion Zimmer Bradley's Ravens of Avalon follows the journey of Boudica, a Celtic princess, and Lhiannon, a priestess who is Boudica's mentor on the Druids' Isle. When the Romans conquer Britain, Lhiannon fights them while Boudica marries a king who has surrendered. Theirs is a great love story, but when he dies, the Romans brutalize Boudica and her daughters. In rage, Boudica rouses the tribes in a quest to drive the Romans from Britain - and the results will seal both her own and Lhiannon's place in history.