In a fanfiction like this, it could be that Percy's powers or abilities were starting to make others jealous. His friends might have felt overshadowed and, in a moment of weakness, turned on him. Also, there could be an outside influence, like a dark force that was manipulating people's minds to make them betray Percy.
Perhaps Percy had made a difficult decision that others didn't understand. He might have sacrificed something important for the greater good, but it was misinterpreted as a selfish act. For instance, he could have given up a chance at a great reward to save a group of strangers, but his friends thought he was being stupid and ungrateful to them for not sharing the opportunity, and so they betrayed him.
One way this could develop is if Naruto's identity as the Jinchuriki of the Nine - Tails is fully exploited against him. People might fear his power and turn on him. For example, the village elders could spread false rumors about him losing control of the Nine - Tails, causing the villagers to shun him. Another possibility is that a powerful enemy uses mind - control or deception to make those close to Naruto, like Sakura or Kakashi, turn against him. This would be a huge blow to Naruto as he values his relationships deeply.
Isolation is a common theme. Percy, being betrayed by everyone, is left all alone. He has to fend for himself and figure out how to deal with the situation without any support. Another theme could be self - discovery. Through this difficult time, Percy may start to learn more about his true self, his strengths, and his limits.
Some writers develop the plot by introducing a third party. This third party might manipulate Thalia into doing something that Percy sees as betrayal. The story then unfolds as Percy tries to figure out what really happened. Writers also show the internal struggle within Thalia if she was forced into the situation. They might describe her guilt and her attempts to make it up to Percy, which adds more depth to the plot.
Percy might initially feel a great deal of anger. He's been wronged by those he thought were his friends. But then, with Annabeth by his side, he could start to focus on finding out the truth. He might use his skills, like his combat abilities and strategic thinking, to figure out who or what is behind the betrayal.
Perhaps Percy's loved ones are made to believe false rumors about him spread by an unknown enemy. As a result, they turn their backs on him. Percy would need to find a way to expose the real culprit behind the rumors and regain the trust of his friends and family.
Perhaps in the fanfiction, an evil version of Goku from an alternate timeline arrives. This impostor causes chaos and destruction, but everyone blames the real Goku. His family and friends are deceived by the look - alike. Goku tries to prove his innocence but no one will listen. He might have to find a way to expose the impostor and regain the trust of those he loves.
Well, one possibility is that Naruto is framed for a serious crime he didn't commit. Maybe someone fakes evidence that he's been working with the enemies of the village. His friends, believing the false evidence, turn their backs on him. He then has to fight not only against the village's distrust but also to clear his name and find out who really set him up.
It could be a jealous rival. This rival might have always been envious of Percy's popularity and skills. So, they manipulate Percy's friends into betraying him to get Percy out of the way and take over his position.
One possible plotline could be that Percy discovers his so - called friends have been secretly working against him all along. Maybe they spread false rumors about him, which makes everyone in his world turn against him. He then has to go on a journey alone to clear his name.
In many such fanfictions, the betrayal of fem Percy can be set up in different ways. Maybe it's a betrayal within the context of the magical world she inhabits. For example, if she's part of a group like the demigods, a fellow demigod could betray her for power or out of jealousy. This then sets off a chain of events where fem Percy has to re - evaluate her relationships and her place in that world.