Zagreus takes one vacation (which ends up accidentally being about four mortal years long because of time dilation in Chaos, but that's not his fault), and when he comes back, Thanatos is missing, the Doors of Death are open, and his previously-unknown demigod half-brother is trapped in Tartarus. Clearly, no one in the Underworld but him can get anything done.

"Usually, people have a lot of questions about these," Chiron says as he raises one of his front hooves even with my bed. "Yet you don't seem nonplussed Mr…?" "Zagreus," I answer. My eyes rove across the wood paneled walls. I hear the whir of talking outside, the clangor of metal, and the twang of a bow. "I don't remember anything. Just a name."

Percy and Annabeth's ill-fated plunge into Tartarus takes a turn when they come upon the underworld's prince and similarly ill-fated god of blood, Zagreus.

[See inside for spoilers/content warnings!] Nico di Angelo, Son of Hades, goes to Gods Councils in stead of his father, because Hades is overloaded. Today the council has been gathered in regards to a prophecy, claiming that Chaos is reforming, and going to destory the world. Except, that can't be right.