Two young witches, once inseparable, are set at odds by secrets and wildly dangerous magic. In the waning days of World War II, with Allied victory all but certain, desperate Nazi diabolists search for a demonic superweapon to turn the tide. A secluded castle somewhere in the south of Germany serves as a laboratory for experiments conducted upon human prisoners, experiments as vile as they are deadly. Across the English Channel, tucked into the sleepy Cumbrian countryside, lies the Library, the repository of occult knowledge for the Société des Éclairées, an international organization of diabolists. There, best friends Jane Blackwood and Miriam Cantor, tutored by the Société’s Librarian—and Jane’s mother—Nancy, prepare to undergo the Test that will determine their future as diabolists. When Miriam learns her missing parents are suspected of betraying the Société to the Nazis, she embarks on a quest to clear their names, a quest involving dangerous diabolic practices that will demand more of her than she can imagine. Meanwhile Jane, struggling with dark obsessions of her own, embraces a forbidden use of the Art that could put everyone she loves in danger. As their friendship buckles under the the stress of too many secrets, Jane and Miriam will come face to face with unexpected truths that change everything they know about the war, the world, and most of all themselves. After all, some choices cannot be unmade—and a sacrifice made with the most noble intention might end up creating a monster.