Deep within the Illyrian war-camps, a dangerous secret has been hidden from the Night Court for years. When Rhysand, Cassian, and Azriel arrive to investigate strange irregularities within the camps, they begin sensing an ancient power buried beneath the mountains—something unfamiliar, watching, and waiting. As tensions rise and violence erupts in the dead of night, the truth threatening to change Prythian forever edges closer to the surface.