Throughout the Fablehaven series, characters are constantly forced to make difficult choices and sacrifices for the greater good. In 'Keys to the Demon Prison', this theme takes center stage as Seth and Kendra realize the true cost of protecting the world from evil. They learn that even seemingly small sacrifices can have a ripple effect, ultimately contributing to the defeat of the Sphinx and his followers.

From the discovery of their magical heritage to their roles in protecting Fablehaven, Seth and Kendra have been on a journey of self-discovery and acceptance of their destinies. In this final installment, they must fully embrace their roles as leaders and harness their unique abilities to unite the magical community. The story emphasizes that true strength lies in recognizing and accepting one's purpose, even when it demands great responsibility.

Facing an enemy as powerful as the Sphinx requires more than just individual strength. 'Keys to the Demon Prison' highlights the power of unity and collaboration. Seth and Kendra must bridge the divides between magical beings, rallying fairies, satyrs, witches, and even reformed demons to fight for a common cause. This theme emphasizes that even in the face of overwhelming odds, working together can overcome seemingly insurmountable obstacles.