Will Henry has been through more that seems possible for a boy of fourteen. He' s been on the brink of death on more than one occasion, he has gazed into hell-- and hell has stared back at him, and known his face. But through it all, Dr. Warthrop has been at his side. When Dr. Warthrop fears that Will' s loyalties may be shifting, he turns on Will with a fury, determined to reclaim his young apprentice' s devotion. And so Will must face one of the most horrific creatures of his monstrumology career-- and he must face it alone. Over the course of one day, Will' s life-- and Pellinor Warthrop' s destiny-- will lie in balance. In the terrifying depths of the Monstrumarium, they will face a monster more terrible than any they could have imagined-- and their fates will be decided.