The Bolt and the HelmEdit
Luke Castellan, a son of Hermes was contacted in his sleep by Kronos, lord of the Titans. The nature of this encounter is unknown, but he was convinced to join Kronos's cause out of a desire for revenge. He was assigned to steal the Master Bolt, Zeus' primary weapon, and the Helm of Darkness, Hades' godly weapon, in order to start a war between the gods.
On a camp field trip to Olympus, he made his move. While the other campers and counselors were asleep he entered the throne room and stole the Master bolt and the Helm of Darkness, as in their arrogance, the Olympians were off guard. They later discovered the theft and went to search for the thief. Ares, son of Zeus, found Luke and fought him. According to Luke, he would have won, but underestimated his opponent and was outmatched. However, he managed to save himself by speaking in Kronos' words that if the Master bolt was successfully stolen, there would be a full scale war. Ares was intrigued and delighted at the idea of a god-war, and spares Luke, but takes the bolt and the Helm with him. Ares later fights Percy Jackson, son of Poseidon, and the latter wins and acquires the two items.
After his defection, Luke took control of a monster cruise ship called the Princess Andromeda, which serves as his base of operations. In order to render Camp Half-Blood defenseless and eliminate a major ally of the Olympians, he poisons Thalia's tree with the help of a spy in the camp's ranks, planning to later 'allow' Percy and Annabeth to take it and heal Thalia's tree, so Thalia, daughter of Zeus will be purged from it. He hopes to use Thalia to fulfill a prophecy made about a half-blood, child of one of the Big Three.
He also frames Chiron for the poisoning, causing him to be fired from the camp and be replaced by Tantalus.
At first, Clarisse daughter of Ares was sent to find the Golden Fleece (and save Grover from the cyclops Polyphemus but was intercepted by Percy Jackson, along with a group of allies consisting of Annabeth Chase and Percy's half-brother Tyson.)
Hermes, Luke's father, persuaded Percy, Annabeth, and Tyson to go on this quest, and told them to go onto the Princess Andromeda, hoping that Percy would meet up with Luke and convince him to stop his assault on the gods. Percy and his allies landed aboard the Princess Andromeda, not knowing of it's true nature, and were captured.
During this time, Luke told the two of his plans to use the Golden Fleece to complete Kronos' resurrection and attempts to recruit Percy and Annabeth to his cause (not willing to recruit Tyson as he reminds him of the cyclops that captured him and Thalia years ago). The two refuse, Luke ordering his bodyguards to feed the trio to a drakon. Percy and the gang escape through a lifeboat propelled by wind canister given to him by Hermes.
Luke later intercepts Percy and his friends when they are looking for a boat that they can use to get back to camp. When he is interrogating Percy for the Fleece in front of a pool (his army of monsters watching), Percy uses the pool to send an Iris message, tricking Luke into revealing the fact that Chiron was framed for the poisoning, resulting in Chiron being rehired (using the fact that he was in a panic due to Percy giving the Fleece to Clarisse). Enraged, he ordered Percy and his friends to be killed. Percy convinces Luke to duel him, a moment both had been waiting for a long time. Luke effortlessly defeats Percy, but is prevented from killing the latter when he's knocked into the pool by a boxing glove glued to an arrow. The arrow was shot by Chiron and his family of centaurs who had just then arrived and proceeded to save them. When Percy escapes, Luke's spite and hatred for the son of Poseidon only increases.
Thalia is resurrected at long last. It is possible and very likely that Luke's allegiance to Kronos was affected after this. He obviously wanted Thalia to be resurrected, shown when he says to Percy that he was going to give him the Fleece after he was done using it to resurrect Kronos.
Search for the OphiotaurusEdit
After Thalia's resurrection, Kronos' second in command, Atlas, devised a plan to capture the Ophiotaurus, which would grant the power to destroy Olympus to whomever killed it and burned its entrails. He remembers the Prophecy and chooses Thalia to be its killer. The attack on Olympus was supposed to happen after Thalia sacrificed the Ophiotaurus.
Luke Castellan is assigned to aid Atlas with this. However, Thalia and Luke were childhood friends and romantically attracted to each other, bringing Luke hesitation to carry this mission out. Knowing this, Kronos tells Luke that if he doesn't succeed in this mission, he would be chosen to be Kronos' host and therefore suffer a fate worse then death.
Some time before his mission in the Labyrinth, Luke journeyed down to the Underworld to perfect his design as Kronos' host. Making his way to the River Styx without being spotted, Luke bathed in the waters, granting him the Curse of Achilles, which made him immune to injury, except in one particular spot.
Luke traveled through the Labyrinth to accomplish this goal. Along the way, he encountered a son of Poseidon who owned a gladiatorial arena in the Labyrinth.
The first time this (Luke bathing in the river Styx) is hinted is in a nightmare of Percy's, where he carries the sky in a plan to trick Annabeth into holding it after she was captured, which of course succeeded. He and Atlas later use this to their advantage by forcing a captive Artemis hold the sky in her place, and Luke is happy that Annabeth survived. He also convinces Atlas to keep Annabeth alive until Winter Solstice. It is a certainty that he bathed in the River Styx before this, as he would not have been able to hold the sky otherwise.
Luke is later seen with Atlas when he summons Skeleton Warriors to hunt down Percy Jackson.
His final appearance in The Titan's Curse is when Percy Jackson, Thalia and Zoë Nightshade, a Huntress of Artemis, arrive at the regrowing fortress of the Titans, where Artemis and Annabeth are held captive. Refusing to have to kill Thalia, Luke attempts to sway her into joining their side. Thalia refuses and the two engage in a duel. Their ferocity and power demonstrated is incredible, but Thalia ultimately wins the duel, disarming Luke and cornering him against the edge of a cliff (she claims that she was always able to beat Luke in a duel). Luke continues to goad Thalia, Annabeth begging her to spare the traitor. As Thalia and Annabeth argue over his fate, Luke attempts to use the opportunity to disarm Thalia, but she kicks him off the cliff, which causes her much guilt later on.
Although his corpse is seen by both an approaching army of monsters and Percy, it is later revealed that Luke survived the fall. This is because Luke had dived into the River Styx, which made him almost invincible.
After her battle with Luke, Thalia pledges herself to Artemis (though she had previously scorned the offer), in order to stop the prophecy from coming true through her. It is possible she was also thinking of Luke when she made this decision.
Mission in the LabyrinthEdit
Due to his failure to kill the Ophiotaurus, Kronos kept true to his word, assigning Luke as his next host. However, before Kronos would take Luke's body, the latter would aid him in planning an invasion of Camp Half-Blood using the Labyrinth, which had an entrance hidden between two rocks which Percy and Annabeth find while playing a game in Camp Half-Blood.
In order to pass the gladiatorial arena in the Labyrinth he had to assign some of his monsters to fight to the death. When we see him, he has to show the giant one more match. Then a captive Percy arrives along with Annabeth and Rachel Elizabeth Dare. As a result, the owner of the gladiatorial matches has Percy face off against three opponents, the first is easily defeated, Ethan Nakamura, who is defeated with some effort and the giant himself, whom is defeated by Percy. As a result, Luke sends his monsters after them, but they escape with the help of Quintus' hellhound, Mrs. O'Leary.
It is revealed that during this time Oceanus begins to make war with Poseidon, incapacitating him when the other gods fight Typhon tell the last battle.
When the time has come, Luke is somehow put into a trance. Two telekhines, along with Ethan Nakamura, perform a ritual to resurrect him. Percy arrives to stop the ritual, but it is too late. When Ethan pledges allegiance to Kronos, the Titan lord rises in Luke's body and is given the reforged weapon, Backbiter, his scythe with which he killed his father. However, Percy is rescued by his comrades, Nico di Angelo sealing their survival by summoning a boulder over the fortress where the ritual was performed. Also saving camp half-blood by stopping him from leading the charge.
A Hero's DeathEdit
The largest part of the war, The Battle of Manhattan, Luke is completely controlled by Kronos. In this time, Percy learns that when Kronos achieves full power inside of Luke's body, he will burn through Luke's body and attain his true immortal form, which would mean any mortal that is in the vicinity will be destroyed like them looking at a god. it would then empower the other titans giving them ultimate power.
Kronos made his way to the throne room of the gods with Ethan Nakamura, destroying the city of Mount Olympus as he did so. Meanwhile Percy, Annabeth, Grover, and Thalia follow closely after. Thalia's legs are trapped under falling debris saving Annabeth from Hera's curse and the others continue on. Kronos battles them in the throne room when they arrive, but Annabeth is able to catch Luke's attention, reawakening him. But only partially.
Ethan Nakamura betrays Kronos and attacks him, unsuccessfully of course, as his sword shattered against Luke's invulnerable body. Several shards lashed back at Ethan and impale him, and he staggered to the ground. Kronos, now having an internal struggle with Luke, opens a hole in the floor beneath Ethan, and he tumbled to his end. Kronos continues to batter the trio, getting closer to his ultimate transformation to his original self.
But Annabeth managed to coax Luke back to his own mind. Luke let Percy knock Backbiter into the hearth, leaving Luke defenseless. A beaten Annabeth gives Percy her knife, as they both realized that Luke was the true hero of the prophecy. Percy gave the knife to Luke who stabbed himself in his Achilles heel, his left armpit.
Percy, Annabeth, and Grover looked away as there was an explosion, and Kronos' fading screams. Luke was left lying on the floor, dying. Annabeth says that he would go to Elysium and Luke replies, "Think … rebirth. Try for three times. Isles of the Blessed", Annabeth replying that he always pushed himself too hard. Then, Luke asks Annabeth if she loved him, and Annabeth replied that she thought she did but truly thought of him as a brother. Then Luke pleads to Percy to not let this happen again. With that Luke died just before the gods arrived after defeating Typhon thanks to Poseidon returning defeating Typhon. In the end he was taken away by the Fates, blessed by Hermes, and remembered and honored as a hero by not only Percy, Annabeth and Grover, but by all.
Thus concludes the Second Olympian War.
Percy, son of Poseidon, child of the prophecy, is given the gift of becoming a god, but declines due to his love for Annabeth, who is finally able to tell her feelings too. He instead requests that the gods claim their children by thirteen and that Camp Half-Blood construct cabins for the minor gods and Hades, and that the peaceful Titans(and possibly Demititans) should be released. He was thinking of Calypso, a kind, beautiful Titan that was confined because of her father Atlas, but all the other peaceful Titans were also released