|Son of Apollo|
|Appearances||The Demigod Diaries|
–Halycon, before dying in The Demigod Diaries
Halcyon, also known as "Hal," was born with ability to see the future. Apollo warned him to never talk about his predictions as it would anger the gods. But he met a young girl who owned a dagger who was destined to die in an accident, but Hal saved her by telling her the future. In return for his help, she gave him the dagger to thank him. By telling her the future, the gods were angry and Apollo cursed him. Hal was forced to wear the skin of the Python, who guarded the Oracle of Delphi as a reminder that he wasn't an oracle. He was stripped of his voice and locked inside his childhood home, a mansion in Richmond, Virginia, where he was guarded by the leucrotae who were linked to his thoughts and could speak for him. They kept him alive as bait for future demigods, who would look for a treasure, as a way for Apollo to remind him that his voice will lead others to their doom. After the demigods were eaten, he would keep their food and other supplies they had traveled with.
He decided to create a diary, filling it with his notes and predictions of the future.
One day, Thalia and Luke were led by Amthelia to Hal's mansion. Upon finding out that Hal trapped them as bait for the Leucrotae, they tried to escape before the sun sets, which they eventually did, by making Greek fire, a dangerous substance. Hal however sacrifices himself to the leucrotae. He gave his diary to Luke.
The Diary of Luke Castellan
Now an old man, Luke Castellan and Thalia Grace stumble across his mansion and the leucrotae. He saves them temporarily by calling them up into his room, but explains that soon the leucrotae will get in and devour them, and that the entire mansion is a trap. He feels remorse for them and knows that no demigod can escape his cursed mansion. Despondent and pessimistic of their chances at first, when he discovers that they are the ones to gain the treasure many demigods have lost their lives trying to obtain, he tries to be useful by helping them search for a way to defeat the leucrotae. Once Thalia receives the treasure, a bracelet that turns into Aegis, they discover the previously lost recipe for Greek Fire, and discover that they can create some from ingredients in the room, planning to use it to destroy the leucrotae.
He gives Luke his diary, a dagger he received from the girl he had saved, and makes him promise that he will stand up to gods, something he wish he had done, and to learn from his diary so that Luke will not make the mistakes that he, Hal, made. He decides to sacrifice himself and lures the leucrotae into his bathroom, before setting off the Greek Fire explosion and dying, finally finding peace.
Halcyon is described as a tall old man, about sixty, with spiky gray hair and green eyes, wearing snakeskin clothes made from the python that guards the Oracle.
- ADHD: Like all demigods, Halcyon possesses inborn supernatural battle reflexes and senses that he uses to analyze the fighting style of his opponent.
- Dyslexia: His brain is hard-wired for reading Ancient Greek instead of modern languages.
- Seer: Being a child of Apollo, god of oracles, he has the ability to see into the future.
- Weaponry: Halcyon may have the ability to use ranged weapons, such as a bow and arrow like other children of Apollo.
Halcyon made three known predictions:
- Halcyon predicted a young girl would die in an accident during his youth.
- Halcyon predicted Thalia would survive the attack of the leucrotae and that she'd sacrificed herself to save Annabeth, Grover, and Luke, and that she would be turned into a tree and sleep for years alone. Then, she would be reawakened and return to her physical form. From coming reawakened she would gain immortality due to joining the Hunters of Artemis. Her path was sad and lonely after being returned to the present and she would later meet Jason Grace, her brother she thought she had lost again.
- Halcyon predicted Luke would survive as well, but that his path was hard to see: Halcyon predicted Luke would survive the attack of the leucrotae as well, but couldn't see his path clearly. He only knew that Luke would sacrifice himself to make sure the gods paid attention to their children, he chose to become Kronos, and that he would betray his friends at Camp Half-Blood.
- Halcyon is a Greek name, most of the time considered a girl's name. It means "kingfisher" and "calm", after a fabled bird, identified with the kingfisher, that was supposed to have had the power to calm the wind and the waves while it nested on the sea during the winter solstice. It also means "quiet", which alludes to Hal Green's muteness.