The sun and the star / Rick Riordan, Mark Oshiro.

As the son of Hades, Nico di Angelo 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 with.