Fatal Flaw

Fatal Flaws are mental or physical weaknesses that demigods, humans, and immortals have. These flaws can often cause the downfall of the being. They are particularly dangerous to demigods.

Description
Fatal Flaws are first mentioned in The Sea of Monsters when Annabeth Chase wants to listen to the Siren's song to learn her fatal flaw. She then explains to Percy Jackson what fatal flaws are. Fatal flaws are mentioned frequently throughout the series. Percy's flaw is personal loyalty, which was told to him by Athena in The Titan's Curse. She says it is a dangerous flaw for the hero of The Great Prophecy to possess, possibly because it means he would "sacrifice the world" to save someone he really cares about. Athena also mentions that flaws that have a good motive are usually the most dangerous ones. They are often the same for demigod children of the same godly origin, notably the children of Aphrodite.

Siren's Music
The Siren's music has an oral hypnotic effect. When the Sirens sing, they sometimes reveal your fatal flaw. Flaws can produce an enticing image which is what lures sailors and adventurers to their deaths, either by drowning, being eaten by the Sirens, or by hitting the sharp rocks (and in modern times, mines) surrounding the island. When Annabeth and Percy are passing by their island, she asks him to tie her to the mast so she can hear the song but not escape to her death. However, after hearing the song, she manages to cut herself free with her knife and dives overboard because of the Siren's hypnotic music. When Percy goes to save her, he sees her vision. She thinks she can fix Luke Castellan, and bring her mother and father back together again, essentially. This is due to her fatal flaw, hubris (deadly pride).

Known Fatal Flaws

 * Percy Jackson: His fatal flaw is excessive personal loyalty. He will risk his life for anyone; his friends, family, strangers, and sometimes even his enemies (like Clarisse La Rue when her dad, Ares, was bullying her).
 * Annabeth Chase: Her fatal flaw is hubris (deadly pride). She thinks she can do anything better than everyone, even the gods, such as saving her family (seen in The Sea of Monsters when she heard the Siren's music).
 * Nico di Angelo: His fatal flaw is holding grudges. He will hold a grudge even if it costs him his own life. In The Battle of the Labyrinth, his deceased sister, Bianca di Angelo, tells him that holding grudges is very dangerous for children of Hades.
 * Thalia Grace: Her fatal flaw is ambition. She desires power even though she knows she does not need it.
 * Leo Valdez: His fatal flaw is a feeling of inferiority. He always feels like he's the least important. He covers this up with comedy.
 * Drew Tanaka: Her fatal flaw is vanity (excessive pride in one's appearance, qualities, abilities, achievements, etc). This flaw is most likely shared by many children of Aphrodite.
 * Arachne: Her fatal flaw is hubris (deadly pride). She thinks she can do anything (weaving, in her case) better than everyone else, even the gods. This resulted into Athena turning her into a monster spider.
 * Pandora: Her fatal flaw is curiosity. When she was given the pithos from the gods, her curiosity made her open the pithos, which she was ordered not to do.