Fatal Flaw

A fatal flaw is a flaw or weakness in heroes and Half-Bloods, but other beings can have one including humans, immortals, and monsters. These can often be the downfall of any given Half-Blood/hero, or anybody for that matter. The topic of fatal flaws first shows up in The Sea of Monsters after Annabeth dove overboard because of the Siren's music. She thinks she can fix Luke and bring her mother and father back together, essentially. This is due to her fatal flaw, hubris (pride). She then explains to Percy about fatal flaws and the topic appears over the rest of the series. Percy's flaw loyalty is said by Athena to be a dangerous flaw for the hero of the Great Prophecy, possibly because it means he would "sacrifice the world" to save someone he really cares about. Athena also says that flaws that have a good motive are the most dangerous ones.

Siren's Music
The Siren's music has an oral hypnotic effect. When sirens sing, they sometimes sing about things like fatal flaws. Flaws can produce an enticing image which is what lures sailors and adventurers to their deaths, either by drowning or by being eaten by the Sirens or by hitting the rocks on the island.

Known Fatal Flaws

 * Percy Jackson's fatal flaw is excessive personal loyalty. He will risk his life for anyone; his friends, family, strangers, and sometimes even his enemies.
 * Annabeth Chase's fatal flaw is hubris (deadly pride). She thinks she can do anything better than even the gods, such as saving her family (seen in The Sea of Monsters when she heard the Siren's music).
 * Nico di Angelo's 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, tells him that holding grudges are very dangerous for the children of Hades.
 * Thalia Grace's fatal flaw is her weak resistance for power. She desires power even though she knows she does not need it.
 * Jason Grace's fatal flaw is a sense of duty for the ones he cares about. He also has a strong moral sense of right and wrong.
 * Leo Valdez's fatal flaw is his feelings of inferiority, thinking that he has to prove himself constantly and that he is lesser than anyone else, like Jason Grace.
 * Piper McLean has two fatal flaws that are noticed in her personality. Depression is one of them. Confidence is the other. She thinks that she is not powerful enough to do anything, and she thinks she's not perfect. But with a little more courage and faith in her abilities, she can do anything she wants, and she will become more powerful she becomes, and therefore, more confident.
 * Silena Beauregard's fatal flaw is vanity (excessive pride in ones appearance, qualites, abillites, achievements, etc.) This flaw is most likely shared by many children of Aphrodite.