From the World of Percy Jackson: The Sun and the Star

Demigods Nico di Angelo and Will Solace must endure the terrors of Tartarus in their attempt to rescue an old friend in this thrilling adventure - set in the funny, mythical and action-packed world of Percy Jackson. As the son of Hades, Nico di Angelo has been through so much, from the premature deaths of his mother and sister, to being outed against his will, to losing his friend Jason during the trials of Apollo. But there is a ray of sunshine in his life - literally: his boyfriend, Will Solace, the son of Apollo. Together the two demigods can overcome any obstacle or foe. At least, that's been the case so far ... Now Nico is being plagued by a voice calling out to him from Tartarus, the lowest part of the Underworld. He thinks he knows who it is: a reformed Titan named Bob whom Percy and Annabeth had to leave behind when they escaped Hades's realm. Nico's dreams and Rachel Dare's latest prophecy leave little doubt in Nico's mind that Bob is in some kind of trouble. Nico has to go on this quest, whether Mr. D and Chiron like it or not. And of course Will insists on coming. But can a being made of light survive in the darkest part of the world? And what does the prophecy that Nico will have to 'leave something of equal value behind' mean? Nico will have to face demons both internal and external as his relationship with Will is tested to the core in this standalone adventure featuring two of the most popular characters in the Percy Jackson saga.