Will Henry and Dr. Warthrop have encountered many horrors together, but can Will endure a monstrumological terror without his mentor? In the terrifying depths of the Monstrumarium, they will face a monster more terrible than any they could have imaginedNand their fates will be decided.
In their final horrific adventure, can Will Henry endure a monstrumological terror without his mentor? "Beyond a simple finale, this is a brave statement about the duplexity of good and evil, and the deadly trap in which all of us are snared" (Booklist, starred review).

Will Henry has been through more than 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 hang in the 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.