The Poisoning of Thalia's Tree is a major event in the Second Titan War.
Percy Jackson and the Olympians[]
The Sea of Monsters[]
On the orders of Kronos, Luke Castellan poisons Thalia's Pine Tree with Elder Python Venom which weakens Camp Half-Blood's borders, allowing increasing monster attacks on the camp. In addition, Chiron was fired as Activities Director due to his inability to cure the poison and Zeus' anger at the tree that he had made out of his daughter's spirit being poisoned. Due to being a son of Kronos, Chiron was distrusted by the gods and made for an easy scapegoat, particularly given Zeus' unwillingness to accept that his father was back. Instead, Chiron was replaced with the ghostly Tantalus.
By the time of Percy Jackson and Annabeth Chase's arrival at camp, the border has become so weakened that Colchis Bulls are able to break through the borders and attack, as does a massive flock of Stymphalian Birds later on while Clarisse La Rue leads a border patrol in order to protect the camp.
After learning of the Golden Fleece and its potential to cure the poison, Percy successfully pressures Tantalus into authorizing the Quest for the Golden Fleece. However, Tantalus chooses Clarisse to lead the quest to Polyphemus' Island, forcing Percy, Annabeth and Tyson to sneak after her with the added intention of rescuing Grover Underwood from Polyphemus. In the process, it's discovered that Luke intends to use the Golden Fleece to restore Kronos, although he ominously mentions letting the camp use the Golden Fleece after he's done with it. After sending Clarisse back to camp by herself, Percy is able to trick Luke into confessing to being the one behind the poisoning to Dionysus via Iris Message, exonerating Chiron, who then rescues Percy and his friends with the help of the Party Ponies.
Upon returning to Camp Half-Blood, Clarisse places the Golden Fleece upon Thalia's tree, and it cures the poison and heals the tree, saving the camp. However, it's later discovered that Kronos had a second reason for poisoning the tree: the Golden Fleece's magic proves to be so strong that Thalia herself is resurrected, giving Kronos another chance to control the Great Prophecy.