Luke Castellan is a son of Hermes who currently sides with Kronos. It is unknown if he is alive as Kronos is currently using his body as a shell to live in.
History
Early Life
Luke is a son of Hermes. Not much is known of his early life, but when he was about fourteen, Luke ran away from his mother's home in Westport, Conneticut. Later on, he met Annabeth, Thalia, and Grover Underwood. It is implied that he began to forge a romantic relationship with Thalia at this time. Together, the quartet traveled to Camp Half-Blood but were chased by monsters. They would have escaped, except for the fact that a cyclops stopped them so that monsters could catch up. The cyclops held Thalia and Luke in the air to direct the monsters in their direction, but Annabeth saved them by stabbing the Cyclops in the foot. Thalia sacrificed her life so that Luke, Grover and Annabeth could get into camp without being killed (Thalia was actually transformed into a tree by the gods to save her life). From that point on, Luke lived at camp all year round, likely trying to reconcile with Thalia's "death".
Allegiance to Kronos
link title
Once, Luke was offered a quest during his years at Camp Half-Blood by his father Hermes: to seek a Golden Apple at the Garden of Hesperides. He was honored by this offer, but later began to question it, as it was done before by Hercules. He went on this journey, but failed and was scarred by the dragon guarding the tree, Ladon. He returned to the camp and according to his words, was treated with "pity" by his comrades. It was then that he began to hold a grudge against Olympus and his father, Hermes, who he began to feel didn't love him due to having rarely seen him.
He was later 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 steel the Master Bolt, Zeus' primary weapon, and the Helm of Darkness, Hades' weapon in order to start a war between the Gods.
On a camp field trip to Mount Olympus, he made his move. While the other campers and counsellors 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 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's words that if the Master Bolt was successfully stolen, there would be a full scale war. Ares was awed by this, and spared Luke, letting him escape with the Bolt and the Helm. Ares would later aid Luke by making Percy Jackson believe. Luke then managed to make it safely back to Mount Olympus before anyone noticed his absence, therefore succeeding in his mission.
Another Resort
Although Luke succeeded in his mission, his failure to defeat Ares cost him dearly. Kronos punished him with nightmares, Luke swearing never to fail the malevolent Titan again. This would also cause him to fear Kronos greatly for many of his later years.
He is then told that another camper would be coming to Camp Half-Blood soon, a camper who would take the hidden items of the Gods that were stolen down to Tartarus, therefore ensuring that there would be a war between the Gods. Luke remembered this and found this camper in the form of a twelve-year old boy named Percy Jackson.
A Scheme in Action
Knowing that Percy Jackson was the boy that Kronos was talking about, Luke befriended the boy and was successful in earning his trust. He welcomed him into the Hermes cabin (where children who haven't been claimed yet go), trained him in sword combat (his skills amazing Percy), and provided him moral support for being a newcomer.
One night after a Capture the Flag game, he summoned a hellhound to attack Percy in an attempt to make Chiron, the camp activities director, believe that the camp wasn't safe for Percy. Therefore, Chiron sent Percy, along with Annabeth and Grover, to find the Master Bolt (the Helm of Darkness' theft was unknown at the time) in order to prevent a war. Before Percy went on his quest, Luke gave him a pair of winged shoes, which would actually drag him, along with the Master Bolt, down into Tartarus when he neared it during his journey in the Underworld (it was believed that Hades stole the Master Bolt). However, this failed when Percy gave the shoes to Grover, who managed to escape from the burden when the time came.
Defection From Camp Half-Blood
After Percy discovered that the Master Bolt was in a backpack that Ares gave him (when he neared Hades) and won the Helm of Darkness in a duel with Ares (who was helping Luke the whole time), Percy returned alive along with his two companions, therefore thwarting Kronos' plan.
In order to prevent Percy from interfering again, he took Percy in the woods for a "friendly chat" and some coke. Percy accepted the offer. While talking with Luke, Percy began to slowly see Luke's dark side, as he threw an empty coke bottle in the river and began to look more bitter in appearance. Luke then summoned a pit scorpion to poison Percy. He told Percy of why he joined Kronos and the plan of gaining the two sacred items for Kronos, leaving by creating a portal using his newly acquired sword Backbiter (which was the newly forged form of Kronos' scythe). However the assassination attempt on Percy failed when the scorpion was killed and the wood nymphs brought him back to camp for healing.
Luke's betrayal was a surprise to many campers and counselors, especially to Annabeth (who had romantic feelings for him), who reacted with great sadness at the loss of her childhood friend.
Poisoning Thalia's Tree
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 defenceless and eliminate a major ally of the Olympians, he poisons Thalia's Tree with the help of a spy in the camp's ranks (who has yet to be revealed). He also frames Chiron for the poisoning, causing him to be fired from the camp and be replaced by Tantalus At first, Clarisse 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 his half-brother Tyson.
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. However, Chiron and his family of centaurs arrive and save them. Luke duels Percy, 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 one of Chiron's relatives. When Percy escapes, his spite and hatred for the son of Poseidon only increases.
When the Golden Fleece is returned, Thalia is resurrected at long last. It is possible and very likely that Luke's allegiance to Kronos was affected after this. Whether this is true or not has yet to be revealed. He also hints wavering loyalty when he says to Percy that he was going to give him the Fleece after he was done using it to resurrect Kronos, although he may have been lying for reasons unknown.
Search for the Ophiotaurus
After Thalia's ressurection, 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 it's origins. He remembers the Prophecy and chooses Thalia to be it's killer.
Luke Castellan is assigned to aid Atlas with this. However, Thalia and Luke were childhood friends and romantically attracted to eachother, 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.
It is presumed that he was granted a special power before he enacted this mission. The first time this 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.
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 Zoe 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 forsees this and 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. It is still unknown how, but there was a hint in a preview of The Last Olympian on the UK website. Go there and you shall see.
Mission in the Labyrinth
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 somewhere in Camp Half-Blood.
Luke travelled through the Labyrinth to accomplish this goal. Along the way, he encountered a son of Poseidon who owned a gladiatorial arena in the Labyrinth. In order to pass 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 oppoments, 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.
Kronos' Return
When the time has come, Luke is somehow put into a trance (judging by a wound on his chest, he was likely killed). Two telekhines, along with Ethan Nakamura, perform a ritual to ressurect him. Percy arrives to stop the ritual, but it is too late. When Ethan pledges allegance 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 dome of earth over the fortress where the ritual was performed.
It is unknown whether Luke is dead or alive. However, when Kronos is hit in the eye with Rachel's hairbrush, he reverts to the Luke persona when in pain. This implies that he is likely still alive.
Luke's Future
The Last Olympian shows glimpses of Luke's past as child through the eyes of Percy thanks to the goddess Hestia. It also is revealed that Hermes and his mother had knew of his fate and his destiny to be Kronos' host body. Annabeth is able to convince Luke to come back due a promise he made to her when was seven that he would always protect her and that they were a family. The knife he gave her was given back to him by Percy to expell Kronos and send him back to Tartarus. Luke was struggling for life and died a hero's death. He planned to be reborn and try and recieve Elysium three times.
See Also
The new Percy Jackson book released today