Thalia Grace is the daughter of Zeus, one of the "Big Three" Greek Gods, the other two being Poseidon and Hades. Grover Underwood was meant to watch over her when she ran away from home at ten years old, reaching Camp Half-Blood two years later. On her journey, she met Annabeth Chase and Luke Castellan. They had brief encounters with the Hunters of Artemis, and were almost convinced to join, but got angry with Zoe Nightshade and did not join. When she was twelve, they were all attacked by a herd of monsters and caught by a Cyclops in New York. She was captured, but thanks to Annabeth, freed. They ran, but the monsters they had briefly escaped from caught up to them. As they reached Half-Blood Hill, Thalia sent Luke, Grover, and Annabeth over the boundary line while she faced the monsters. She sacrificed her life for her friends, but her father Zeus took pity on her and turned her into a pine tree to preserve her, keeping the rest of the camp safe.