The Court of the Dead

A few months after returning from the depths of Tartarus, demigods Nico di Angelo and Will Solace agree to join Nico's half-sister Hazel Levesque at Camp Jupiter on the West Coast. She needs their help in managing a situation that the boys inadvertently brought about: the demigods showed the monsters of the Underworld that they have options; they don’t have to be evil.
Now some of those monsters have taken up residence at Camp Jupiter to seek refuge. Nico and Will are on site, assisting Hazel when one by one monsters start disappearing from camp. A mysterious dark force is at work, and its plan is to punish all monsters for their past crimes.