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 .

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Rick Riordan
Richard Russell Riordan Jr. is an American author. He is known for writing the Percy Jackson & the Olympians series, about a twelve-year-old Percy Jackson who discovers he is a son of Greek God Poseidon. 20th Century Fox has adapted the first two books of his Percy Jackson series as part of a series of films. His books have spawned related media, such as graphic novels and short story collections. He conceived the idea for the Percy Jackson series as bedtime stories about ancient Greek heroes for his son Haley. Haley had been diagnosed with ADHD and dyslexia, inspiring Riordan to make the titular protagonist ADHD and dyslexic. Prior to Percy Jackson, Riordan had written the Tres Navarres series, a series of mystery novels for adult readers.