Storm of Echoes - Mirror Visitor Quartet #4 by Christelle Dabos

Ophelia and Thorn, at the center of a great universal game in which the stakes are life and death, arrive at the observatory of the Deviations, an institute shrouded in absolute secrecy and overseen by a sect of mystical scientists who secretly conduct terrifying experiments. There, Ophelia and Thorn hope to put a stop to the destruction and death and bring balance and harmony to their world.