It could be due to some sort of poison or magical spell. In the Percy Jackson universe, there are all kinds of creatures and magic that could cause such a reaction. For example, if he was exposed to a slow - acting venom from a mythical beast, it might gradually make him faint.
Another possibility is that he's experiencing some kind of emotional shock. Perhaps he learned some really disturbing news about his friends or family, and the shock was so great that it made him faint. This is quite common in the series where the characters are often put in situations that test their emotional strength.
Annabeth might find him. She's always been close to Percy and they often go on adventures together. So it's likely that she'd be the one to come across him when he faints.
One possible reason could be extreme exhaustion. Peter Parker has a double life as a student and Spider - Man. He might be overworked, not getting enough sleep, and his body just gives out. For example, he could have been fighting multiple villains in a row without a break.
Perhaps he has been exposed to some sort of deadly and unknown substance in the mythical world. This could be a new type of monster venom or a strange plant that he accidentally ingested. The substance could be attacking his body from the inside, causing his internal organs to bleed, and thus he throws up blood.
Well, perhaps in the fanfic, there's a prophecy that gets misinterpreted. The gods think Percy is going to cause great harm based on this wrong interpretation, so they send him to Tartarus. It could also be that there's some sort of power struggle among the gods. Percy might be caught in the middle of it, and one side decides to get rid of him by banishing him to Tartarus to tip the balance in their favor.
Well, it could be due to some kind of time - related anomaly. In the world of Percy Jackson, there are all sorts of crazy time - based magic. Perhaps a time - traveling experiment by one of the gods or demigods went haywire and Percy got caught in it, reverting him to his childhood. It might also be part of a prophecy. There could be a prophecy that states Percy must relive his childhood for some greater purpose, like learning a lesson he missed the first time around.
Well, in fanfiction, one reason could be the loss of his friends. Percy has been through so many battles, and if he loses someone close like Annabeth or Grover, it could send him into a deep depression. Another reason might be the burden of his prophecies. He's constantly expected to save the world, and that kind of pressure can wear a person down over time.
Perhaps Percy has proven his worth through his unwavering loyalty and courage. Loki, who is used to seeing self - serving and deceitful behavior among the gods and mortals, is impressed by Percy's pure - heartedness. His actions of always protecting his friends and standing up for what is right might make Loki feel that he is worthy of respect, leading to Loki bowing to him.
Maybe Percy unknowingly breaks some ancient and very strict laws of the Gods. For example, he might interfere with some sacred rituals or places that are off - limits to mortals, even demigods like him. In the fanfiction world, the Gods could be very unforgiving when it comes to such violations and decide to kill him as a result.
One possible reason could be that Percy has shown some extraordinary power that even Thor, a powerful god, respects. Maybe Percy has defeated a common enemy in a really remarkable way that makes Thor recognize his strength and kneel in admiration.
One possible reason could be past trauma. Maybe he had a bad encounter with a man in the past, like a bully or an authority figure who mistreated him. This could have left a deep scar, making him terrified.