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|“||You must retrieve the sword before it leaves our realm. Or all is lost.||”|
Luring Percy Jackson, Thalia Grace, and Nico di Angelo to the Underworld, Persephone instructs the three demigods to track down a thief who has stolen the newly forged sword of Hades, a weapon with the power to imprison or release souls from the Underworld. She bestows a tracking flower that will aid them in finding the thief before he finds a way to escape to the sealed Underworld.
Journeying through the Fields of Punishment, the demigods come across Sisyphus rolling his boulder up a hill. Believing the three of them to be the Furies in disguise, Sisyphus refuses to talk until Thalia offers to take his burden while Percy and Nico question him. Under threat from Nico, Sisyphus reveals that he advised the thief to seek Melinoe for a way out of the Underworld. He attempts to flee after tricking Percy and Nico, but an invisible leash keeps Sisyphus from running away. Thalia loses her grip on the boulder and sends it back down to Sisyphus, who once again assumes the burden of rolling it up the hill.
On their way to Melione's cave, Percy suspects the thief to be Ethan Nakamura. They are suddenly surrounded by Keres, battlefield spirits who feed on violent death. The demigods battle the evil daimones, but Percy is wounded in the midst of battle. After vanquishing the remaining Keres, Thalia and Nico try tending to Percy's wound, but all they manage is to slow down the infection.
Resuming their quest, the demigods make it to the River Lethe near the outer rim of the Underworld. Needing a way across, Percy uses his powers to lift the river from its bank, clearing a path for them to cross. However, the strain from holding the river up so long weakens Percy as he walks across and causes it to crash down on him. Fortunately, he proves to be immune to the mind-erasing properties of the water and staggers out of the River Lethe before collapsing.
Reviving Percy with nectar, the demigods enter Melinoe's cave. The goddess appears in the guise of Thalia and Nico's mothers, but Percy breaks her illusion. Melinoe reveals that they're too late, for she has already struck a deal with Kronos. Thalia fires an arrow at the goddess, who simply disappears as the last petal on the tracking flower wilts away.
Standing at the mouth of the cave are Ethan Nakamura and Iapetus. Although Ethan tells the Titan to flee, Iapetus charges at the demigods with the intent of killing them. Nico summons an army of skeleton warriors to distract Ethan while he, Thalia, and Percy fight Iapetus. As Percy is flung close to the River Lethe by Iapetus, he taunts the Titan and drags Iapetus into the river with him, thus erasing the latter's memories. After pulling him out, Percy convinces Iapetus that his name is Bob and that he is a friend of theirs. Ethan Nakamura escapes through Melinoe's cave after Thalia stops him from lunging for the sword of Hades.
After Bob heals Percy's shoulder wound, the Furies arrive to take the three demigods and Bob to the palace of Hades. There, it is revealed that Persephone had the sword illegally forged against Hades' direct orders. Hades orders Nico, Thalia, and Percy to not speak of it to anyone, and hands the sword over to the Furies to have it finished. After he vanishes, Persephone presents three travel roses to the demigods so that they can return to the upper world. She disappears after Percy threatens to have Bob throw her in the River Lethe for manipulating them into procuring the sword.
Nico decides to remain in the Underworld to look after Bob. He reminds Percy of his offer to fight Kronos before departing. Thalia then suggests they grab a cheesburger, and together she and Percy return to the upper world via their travel roses.
- This is the second quest in the Percy Jackson and the Olympians series to have members comprising of demigod children of the Big Three. The first is the Quest for Artemis, though one member was not discovered to be a child of the Big Three until the quest's completion.
- As Percy mentions at the beginning of Percy Jackson and the Sword of Hades, the quest takes place on Christmas.