Aisling, a goddess in human form, was born to rule both domains and — with her twin, Anya — unite the Celts with the powerful faeries of the Middle Kingdom. But within medieval Ireland interests are divided, and far from its shores greater forces are mustering. Jordan, the Vatican commander tasked with vanquishing the remnants of otherworldly creatures from a disenchanted Europe, has built a career on such plots. But increasingly he finds himself torn between duty and his desire to understand the magic that has been forbidden. As kings prepare, exorcists gather, and divisions widen between the warring clans of Ireland, Aisling and Jordan must come to terms with powers given and withheld, while a world that can still foster magic hangs in the balance.