The camp leaders might betray them. If there is a power struggle within the camp, and Percy and Nico are seen as threats to the current leadership's power, the leaders could set them up. They could make it look like Percy and Nico are breaking camp rules or endangering the other campers, which would lead to the rest of the camp turning against them.
In a 'Percy and Nico are betrayed' fanfiction, perhaps they are betrayed by the gods. The gods, fearing their growing power, might spread lies about them among the demigods. Percy and Nico then have to fight not only against the false accusations but also the demigods who turn on them. They would have to find a way to clear their names and prove their loyalty.
It could be a jealous demigod. Maybe someone who is envious of Percy's popularity and Nico's unique powers. This person might see an opportunity to get rid of them and take their place in the demigod hierarchy.
In a fanfiction about their betrayal, it could be that they are betrayed by a new character who infiltrates their group. This new character might seem friendly at first but then turns on them to further their own secret agenda, perhaps related to an ancient prophecy. Percy, Nico and Thalia would then have to figure out how to stop this new threat while also clearing their names.
One possible plotline could be that Percy and Nico are betrayed by a close friend within the demigod community. Maybe they are framed for a crime they didn't commit, like stealing a powerful artifact from the gods. This could lead to them being hunted by other demigods while they try to clear their names.
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.
Well, they could be betrayed by someone they trusted deeply, like a fellow demigod. Maybe this person spreads false rumors about them, leading to their isolation within the demigod community. The three of them would then have to struggle to clear their names and find out who really set them up.
Well, they might lose trust in their friends. For example, if they were betrayed by someone they considered a close ally in the demigod world, like a fellow camper at Camp Half - Blood. Percy could become more withdrawn, as he has always been a bit of a loner at heart. Thalia might turn bitter and angry, using her skills as a hunter more aggressively. Nico, being the son of Hades, could potentially be pushed towards using his darker powers in retaliation, perhaps even seeking vengeance in the Underworld's way.
Percy might be angry at first. He's used to being a loyal friend, so betrayal would hit him hard. He'd probably try to find out the truth and clear his name. Jason, being a more diplomatic type, might initially be shocked and then try to reason with those who betrayed him. Nico, on the other hand, could retreat into himself. He has a more solitary nature, and the betrayal might make him even more withdrawn.
One possible reason could be jealousy among other characters. Maybe there are those who envy Percy's powers and leadership, Jason's status as a hero, or Nico's unique abilities. Another reason might be a misunderstanding. For example, they could be wrongly accused of a crime they didn't commit, which leads to others turning against them.