Obsidian Equinox - The Celestial Shadow Series, #4

The Shadow was born in the dark. The Sun must follow it there.
Highland Home is in ashes. Tawnya has been rescued but stripped of her magic, and The Astral Plane is no more. Mairin Cote is no longer fighting for territory; she is fighting for the truth. A map found on the body of the Serpent Keeper High Elder points to a single, terrifying location: The Black Forest, Germany. The ancestral cradle of the Auer-Schlange.
Mairin, her silenced mother Loraine, and a powerless Tawnya must cross the ocean to infiltrate the Old World—a landscape of ancient, predatory magic governed by the ruthless European Council. They are hunting for the First Vow, the original blood-spell that bound the Auer family to their curse.
But they are not the only ones drawn to the source. Kathryn and Mathias are tearing a bloody path across Europe, leaving destabilized ley lines and natural disasters in their wake. Mathias is fracturing, his soul caught between Kathryn's command and Mairin's love.
In the forgotten catacombs beneath Berlin, Mairin discovers a secret that shatters her own identity: her father's death was no accident, and his bloodline carries a power that rivals the Auer curse. As the Obsidian Equinox approaches—a time when the veil between the living and the spirit world is thinnest—Mairin must enter Mathias's dreams to save his sanity, only to find that to break the Vow, one of them may have to die.