The House of Hades (The Heros of Olympus, Book 4)

The fourth book in the Heroes of Olympus series, now featuring cover art by graphic novelist Nilah Magruder. After Annabeth and Percy fall into a pit leading to the Underworld at the end of The Mark of Athena, the remaining five demigods must follow Percy's instructions to locate the mortal side of the Doors of Death. They must fight through Gaea's forces while Percy and Annabeth navigate the House of Hades. If all seven demigods succeed, they can seal the Doors from both sides and prevent the giants from raising Gaea. However, a critical problem emerges: if the Doors are sealed, how will Percy and Annabeth escape? The demigods have no choice but to proceed. If they fail, Gaea's armies will never die. Time is running out—the Romans will march on Camp Half-Blood in about a month. This adventure takes the characters into the depths of Tartarus, raising the stakes higher than before.