Meanwhile, in Alaska, the giant Alcyoneus has captured Thanatos, the god of death, and has prevented the souls of the dead from passing on. This causes the rise of the undead, who are loyal to Gaea, the earth mother. Gaea is also awakening her other children, the giants, who are destined to fight the gods. She plans to use the blood of two demigods, one Greek and one Roman, to awaken her fully during the Feast of Fortuna, a festival that celebrates the goddess of luck.
Percy, Hazel, and Frank are chosen by Mars, the god of war, to go on a quest to free Thanatos and stop Gaea. They are given a prophecy that says:
Go to Alaska. Find Thanatos and free him. Come back by sundown on June twenty-fourth or die.
They also receive a gift from Mars: a piece of wood that can turn into any weapon, but can only be used by Frank. Frank is told that his life is tied to the piece of wood, and that he will die when it burns up.
The trio leaves the camp and heads to Alaska, facing many dangers and enemies along the way. They encounter Iris, the goddess of the rainbow, who gives them a jar of wind to help them fly. They also meet Phineas, a blind seer who has a harpy named Ella, who can recite prophecies and books from memory. Phineas challenges them to a contest of finding the true source of the gorgons’ blood, which he needs to restore his sight. Percy wins the contest by using his water powers, and Phineas agrees to tell them where to find Alcyoneus. He also reveals that one of them will die during the quest.
The trio continues their journey and reaches Alaska, where they find Alcyoneus and his army of undead. They also discover that Hazel’s brother, Nico di Angelo, a son of Hades, has been captured by the giant and is being used as a bait for Percy. Hazel, who has the power to manipulate the metals and jewels of the earth, decides to face Alcyoneus alone, while Percy and Frank free Thanatos and Nico. Hazel manages to trick Alcyoneus into leaving his home territory, where he is invincible, and defeats him with the help of Arion, a super-fast horse that she befriends.
