In 1818, Cillian Valour, an Irish rebel's son with limited prospects, is determined to marry his longtime love, Martha Ashworth, daughter of a local landlord. When her father forbids their engagement, Cillian turns to the heavens for help in an act of desperation-but it is a mysterious stranger that answers his call. After accepting an offer too good to refuse, Cillian and Martha are forever changed. Together, the star-crossed lovers turn away from the light and cut a swath of horror and misery across the land-until one night in Rome, the unthinkable happens.

Emily Corvo's life in modern-day Boston is upended by the murder of her mother and the disappearance of her father. Learning that her mother was callously slaughtered by something not of this world, Emily vows to rescue her father and avenge her mother by hunting down those responsible. But she may find that only by trusting the enemy can she quench her thirst for vengeance.