«The Ouroboros Cycle, Book Three: A Long-Awaited Treachery» by G.D.Falksen

For a thousand years, the immortal Shashavani have hidden from the world in the halls of their mountain fortress, amassing great stores of knowledge in a haven of academic tranquility. And while Doctor Varanus has never found tranquility much to her taste, even she cannot deny that the peace and quiet make for a pleasant diversion following the chaos of her recent adventures in London. Having left the horrors of bloodshed and wellness behind her, Varanus has thrown herself headlong into the study of the Shashavani condition, determined to learn the secrets of their undying power.
But all is not as it seems in the House of Shashava. As winter snow covers the Shashavani valley and chokes the mountain passes, sinister events are unfolding in the shadows. Whispers of conspiracy echo through the halls. Soon loyalties will be tested and friendships betrayed. Blood will flow. For upon the frozen steppe, an ancient evil stirs and turns its gaze toward the House of Shashava, hungering for power and revenge.
As darkness descends, one question remains:
Do you serve the Winter King?