Thalia is a young demigod who, eight years before the novel begins, began making her way to Camp Half-Blood with Grover as her keeper. Disaster struck, however, when Thalia, Grover, Luke, and Annabeth—who joined the group after running away from home—were attacked on Half-Blood Hill. Though Grover’s instructions were to only rescue Thalia, he followed Thalia’s orders and saved Luke and Annabeth, leaving Thalia as a sacrifice. Thalia then took the form of a huge pine tree on the top of the hill. Thalia suffered this fate because she is Zeus’s daughter, and Zeus broke his vow to stop fathering children after World War II. Thalia and the tree stand as reminders of how dangerous the mortal world outside of Camp Half-Blood is for demigods, especially those children of the Big Three.