The House of Hades (The Heroes of Olympus #4)

In the fourth installment of The Heroes of Olympus series, The House of Hades, Annabeth and Percy are thrown into the Underworld after a dangerous fall at the end of The Mark of Athena. With their friends grieving and in shock, the remaining five demigods must push aside their emotions and follow Percy's instructions to find the mortal side of the Doors of Death. If they can successfully battle through Gaea's forces and if Percy and Annabeth can survive the treacherous House of Hades, then the Seven will have the chance to seal the Doors on both sides and prevent the giants from resurrecting Gaea. However, Leo raises a valid concern - if the Doors are sealed, how will Percy and Annabeth be able to escape? But with no other options, the demigods must press on. Time is of the essence as the Romans plan to attack Camp Half-Blood in just a month. The stakes have never been higher as this adventure takes us deep into the depths of Tartarus.