House of Hades (Heroes of Olympus Book 4) - Rick Riordan

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. Meanwhile, Hazel and the crew of the Argo II have a choice: to stop a war or save their friends. Whichever road they take one thing is certain - in the Underworld, evil is inescapable.