Rick Riordan, Mark Oshiro: The Sun and the Star

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So when a desperate voice starts plaguing his dreams, Nico is convinced it's coming from the Underworld and belongs to an old friend - a reformed Titan called Bob. Then an ominous prophecy leaves Nico in no doubt - Bob needs his help and Nico must rescue him.
Of course Nico's boyfriend, Will Solace, the son of Apollo (the god of light) insists on joining the quest too. But can will even survive in the darkest part of the world? And what does the prophecy mean when it says that Nico will have to leave something of equal value behind?