Thalia's tree

History of Thalia's tree
5 years before Percy got to Camp HalfBlood, Thalia, Annabeth, Luke, and Grover were traveling to Camp HalfBlood. She was the daughter of Zeus. That was considered an outrage considering the fact that the Big Three made an oath to never marry a mortal woman again. Hades was evil as always and sent hundreds of monsters after them. Meanwhile Grover, as said in the book, made a wrong turn and reached Camp HalfBlood late. The monsters were only a few 100 yards behide them. Before they reached the camp's safety,the monsters attacked. Thalia told Luke, Annabeth, and Grover to run while attacked the monsters. It was Thalia the monsters wanted so they ran for it. Several minutes later, Thalia was finally struck down and she died. Zeus pitied her and turned her into a tree. From that day on, that represented the camp's border. It fended off all monsters and mortals. The only way for a monster to get in is to be summoned in( In the Book 2 The Sea of Monsters, Luke poisoned Thalia's tree causing chaos within Camp HalfBlood).

Death of a tree and rebirth of Thalia
In The Sea of Monsters, Percy, Annabeth, and Tyson go looking for the Golden Fleece, the only cure for Thalia's tree. Coincidentally, Grover found it while looking for the Great God Pan. Once they were back at Camp HalfBlood, they brought the Golden Fleece over to Thalia's tree and all the suddenly, the tree disappeared and a girl about 12 years old was laying there. Her name was Thalia, Daughter of Zeus.