In this fourth installment of the best-selling Heroes Olympus series, the Greek and Roman demigods from the Prophesy of Seven must work together to seal the Doors of Death-and help Percy and Annabeth escape the Underworld in the process. At the conclusion of The Mark of Athena, Annabeth and Percy tumble into a pit leading straight to the Underworld. The other five demigods have to put aside their grief and follow Percy's instructions to find the mortal side of the Doors of Death. If they can fight their way through the Gaea's forces, and Percy and Annabeth can survive the House of Hades, then the Seven will be able to seal the Doors from both sides and prevent the giants from raising Gaea. But, Leo wonders, if the Doors are sealed, how will Percy and Annabeth be able to escape? They have no choice. If the demigods don't succeed, Gaea's armies will never die. They have no time. In about a month, the Romans will march on Camp Half-Blood. The stakes are higher than ever in this adventure that dives into the depths of Tartarus. Contributors: Rick Riordan (www. rickriordan.com) is the author of the # 1 New York Times best-selling The Heroes of Olympus, Book One: The Lost Hero; The Heroes of Olympus, Book Two: The Son of Neptune; The Heroes of Olympus, Book Three: The Mark of Athena; the #1 New York Times best-selling Kane Chronicles; and the five books in the #1 New York Times best-selling Percy Jackson and the Olympians series. His previous novels for adults include the hugely popular Tres Navarre series, winner of the top three awards in the mystery genre. He lives in San Antonio, Texas, with his wife and two sons.