Heroes of Olympus - The House of Hades (Heroes of Olympus Book 4)

The stakes have never been higher. If Percy Jackson and Annabeth fail in their quest, there'll be hell on Earth. Literally. Wandering the deadly realm of Tartarus, every step leads them further into danger. And, if by some miracle they do make it to the Doors of Death, there's a legion of bloodthirsty monsters waiting for them.