A Drop of Night by Stefan Bachmann

Modern-day teenagers meet a palace of terrors locked up since the French Revolution in this surprising and haunting thriller from Stefan Bachmann, the internationally bestselling author of The Peculiar and The Whatnot. A Drop of Night will thrill fans of Neal Shusterman and Jessica Khoury.
Seventeen-year-old Anouk has finally caught the break she's been looking for-she's been chosen to participate in an exclusive program that includes an all-expense-paid trip to France and a chance to explore the hidden underground Palais des Papillons, or Palace of Butterflies. Along with four other gifted teenagers, Anouk will be one of the first people to set foot in the palace in more than two hundred years. Bachmann's masterful scene-building alternates between Anouk's flight through the palace and the struggles of Aurelie, who escaped the French Revolution by fleeing into the Palais des Papillons in 1792.