Thalia's Pine Tree is the pine tree on the very tip of the camp. As Thalia Grace laid dying after her wound from her fight to save her friends, her father Zeus, God of the skies, Ruler of the Gods, created Thalia's pine tree to not only save his daughter but to stop that from happening again. He created a barrier to protect lost demigods and to keep Camp Half-Blood a safe place from the monsters trying to kill them.
Five years before Percy Jackson arrived to Camp Half-Blood, Thalia Grace, a daughter of Zeus, Annabeth Chase, a daughter of Athena, Luke Castellan, a son of Hermes, and Grover Underwood, their satyr protector, were traveling to Camp Half-Blood.
Thalia's birth was considered an outrage considering the fact that the Big Three made an oath to never sire demigod children. Since Zeus broke this oath, Hades was furious and sent a whole army of monsters from the Underworld, including the Furies, after them. Grover told Percy in The Lightning Thief that he made a couple of wrong turns (one of the worst was towards a Cyclops' lair in Brooklyn) and reached Camp Half-Blood too late. The monsters were only a few yards behind them and, therefore, they were attacked before they could reach the safety of Camp Half-Blood.
Thalia told Luke, Annabeth and Grover to run while she confronted the monsters. At Thalia's request, the others ran for the borders of Camp Half-Blood. During the battle, Thalia was mortally wounded. As she lay dying, Zeus took pity on his daughter and turned her into a pine tree to preserve her soul so she would not go to the Underworld and face the wrath of Hades. From that day on, her pine tree represented the camp's border, and gave the hill its name: Half-Blood Hill. It fended off monsters and mortals with the magical border around the camp.
The only way for a monster or mortal to get in is to be summoned inside the camp or to be invited into the camp as shown in The Sea of Monsters when Annabeth allowed Tyson, Percy's Cyclops half-brother, into the camp just so he could help Percy defeat the bronze Colchis bulls that were invading Camp Half-Blood and also in The Last Olympian where Percy is shown a vision in which May Castellan is invited to take the job of the Oracle.
Percy, Annabeth, and Tyson run off to go looking for the only cure for Thalia's tree, the Golden Fleece, after it had been poisoned by Luke. Coincidentally, Grover found it while looking for the god of the wild, Pan. After their adventure in the Sea of Monsters, they put the Golden Fleece on Thalia's tree. Immediately, the tree begins to heal and expel the poison, but it worked too well. Several days later, while Annabeth was on watch (because there was no guardian yet), Thalia was expelled from the tree, having only aged at the rate of the tree. However this was Kronos' plan all along to give him another chance to control the Great Prophecy.
|Camp Half-Blood:||Amphitheatre | Archery Field | Bunker 9 | Cabins | Camp Borders | Camp Half-Blood Forest | Climbing Wall | Cave of the Oracle | Dining Pavilion | Grove of Dodona | Half-Blood Hill | Rec Room | Sword Fighting Arena | Thalia's Pine Tree | Myrmeke's Lair | Strawberry Fields | The Big House | The Big House Attic | Zephyros Creek ||