Even old enemies of the demigod world, who see Percy and Jason as threats, could be the antagonists. They might use deception and betrayal as a way to weaken the demigod forces, knowing that Percy and Jason are important figures among them.
It could be the traditional villains from the original story, like the Titans or some powerful monsters. They might torture Percy to get information or just to cause chaos.
It could be the traditional villains from the original story. Maybe some of the old enemies of Percy who have a vendetta against him. For example, the Titans or some rogue demigods who oppose him for power or personal reasons.
In many Percy, Nico, and Jason - related fanfictions, the idea of betrayal can vary greatly. It could be that in some stories Jason betrays Percy out of loyalty to a different group or a misunderstanding. For example, if there's a power struggle within a camp and Jason believes Percy's actions might endanger the whole camp, he might take actions that Percy perceives as betrayal. However, it could also be Nico who is betrayed by the other two if they accidentally expose his secrets that he's been keeping to protect himself and others.
Obviously, Percy Jackson is the central character. The Olympians such as Zeus, Poseidon, and Athena are likely to be involved as the ones doing the betraying. Other demigods like Annabeth Chase could also be part of the story. She might be torn between her loyalty to Percy and her respect for the Olympians. There could also be minor gods and goddesses who either side with Percy or the Olympians, depending on their own motives and relationships. For example, Hades might see an opportunity to gain more power if Percy is out of the picture, or he could help Percy out of a sense of fairness.
One possible antagonist could be an ancient and powerful Titan who has escaped from Tartarus. This Titan is bent on revenge against the gods and sees Percy and Jason as the key to getting to the Olympians. He uses his dark magic to create all sorts of disasters and tries to turn Percy and Jason against each other. Another could be a group of rogue demigods who have turned to the dark side. They envy Percy and Jason's status and friendship and try to undermine them at every turn.
It could be an old enemy from their past, like a resurrected Titan or Giant. These powerful foes might manipulate Percy and Jason into betraying their friends as part of a grand plan to regain power or take revenge on the demigods.
It could also be that the Olympian Council as a whole acts as the antagonist. They might have made decisions that Percy sees as a betrayal, such as sacrificing the safety of demigods for their own political gain. Each god on the council, with their own self - interests, might contribute to Percy's feeling of betrayal, making the council as a whole the main force he has to oppose.
A possible betrayer could be a jealous demigod. There are always those in the Percy Jackson world who envy Percy's abilities and the attention he gets. This demigod might see an opportunity to get rid of Percy and make themselves look better in the eyes of the other campers.
The gods, especially Zeus. He has always been a bit suspicious of Percy. In fanfiction, he could easily be written as spreading lies about Percy to the other gods and demigods, leading to everyone turning against him. Also, some new characters introduced in fanfiction might be the ones who start the chain of events that lead to Percy's all - around betrayal.
One character that could betray Percy is Thalia. She has her own issues with the gods and her place in the world. Maybe she gets jealous of Percy's growing popularity and power among the demigods. So, she decides to side with an enemy to take Percy down a notch. Additionally, some minor characters that Percy has helped in the past could be the ones to betray him. They might feel indebted to him but also resentful at the same time. An enemy could manipulate their feelings and turn them into traitors against Percy.