Nico di Angelo, hijo de Hades, ha superado las pruebas más difíciles que podría haberse imaginado: desde la muerte de su madre y su hermana hasta perder a su amigo Jason durante los juicios de Apolo. Pero ahora hay un rayo de sol en su vida: Will Solace, el hijo de Apolo. Juntos, los dos semidioses pueden superar cualquier obstáculo. Al menos así fue hasta el día de hoy. Ahora una voz llama a Nico desde el Tártaro, la parte más baja del Inframundo. Él tiene clara su misión, pero Will no va a dejar que vaya solo… La pregunta es: ¿puede un ser hecho de luz sobrevivir en la parte más oscura del mundo? ¿Podrá vencer los demonios que lo rodean y los que tiene en su interior? 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 must go on this quest, whether Mr. D and Chiron like it or not. And of course, Will insists on coming with. But can a being made of light survive in the darkest part of the world? And what does the prophecy mean by Nico will have to "leave something of equal value behind?"