The Daughters of Ys #1

While Princess Rozenn spends her life wandering the countryside, Princess Dahut works magic to sustain the City of Ys... notably by seducing and beheading a series of young men. When she finally lets one go, a mysterious stranger declares their contract broken and demands the penalty, which is destruction of Ys. Rozenn rescues her father and sister, but Gradlon kills Dahut. He rules from Brittany's ancient capital, and Rozenn succeeds him at his death.