From The World Of Percy Jacson - 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. Hazel needs their help in managing a situation that the boys inadvertently brought about in The Sun and the Star, where 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. Things only get worse when Nico, Will, and Hazel learn that they’re all connected to it . . .