Once a Goddess - Brigid of Ireland, #3

The prequel... For the sake of peace, Brigid of the Tuatha dé Danann enters into an arranged marriage with Bres, the next chieftain of the enemy Fomorians, whose weapons and brute strength challenge Danann magic. Brigid is told to spy for her people and keep the source of their powers secret - dangerous tasks that complicate her desire to make the best of her forced union.  Scared, but determined to make the marriage work to save her people from annihilation, Brigid faces the Fomorians alone.
She begins to suspect that her husband is breaking the truce through lies and political manipulation and she soon discovers betrayals on both sides. When Bres's tyranny threatens the very existence of the Danann, Brigid must risk her life to unseat him from power. Set in a time when myths were reality, Once a Goddess brings the legend of Ireland's magical Tuatha dé Danann to life.