Crimson Zenith. The Celestial Shadow Series, #5

The Vow is broken. The world is bleeding. In the catacombs of Berlin, Mairin Cote achieved the impossible: she shattered the Altar of the First Vow. But victory has come at a devastating cost. Mathias and Kathryn have been captured by the European Council, imprisoned in The Citadel-a fortress high in the Swiss Alps designed to hold the world's most dangerous magical entities. But The Citadel cannot hold Kathryn.
The Huntress is no longer just a vampire; she is a walking magical reactor in meltdown. Corrupted by the sheer volume of power she stole from the Serpent Keepers, Kathryn has initiated the Serpent's Swallow-a forbidden, apocalyptic ritual that will drain the earth of all magic, killing every supernatural being in existence to silence her own pain. The countdown to the Crimson Zenith has begun. To stop the end of her world, Mairin must do the unthinkable: she must unite the scattered Sentinels, the remnants of the Serpent Keepers, and the ruthless European Council into a single army.
Leading this volatile coalition, Mairin marches on the Alps to face the woman who was once a victim, but is now the ultimate villain. In the thin air of the summit, amidst a storm of wild magic, Mairin will find Mathias one last time. But he is no longer the Weapon or the Soul-Drifter. He is a man facing a final choice: watch the world burn, or sacrifice his eternal life to extinguish the flame he created.