Well, perhaps Percy could be disillusioned with his current situation in his normal world. He might see the chaos army as a way to break free from the constraints he's felt. He could start off as an outsider in the chaos army, having to prove himself.
The plot could involve Percy being lured into the chaos army by some false promises. He then realizes the true nature of the chaos army which is far more complex and dangerous than he thought. And he has to decide whether to stay or find a way to escape while also dealing with the new powers and friends he's made there.
One possible plot could be that Percy, with his battle experience from the Greek world, quickly rises in the ranks of the Chaos Army. He might bring new strategies inspired by his demigod training, like using the element of water in unexpected ways during battles. His loyalty and sense of justice could also lead him to take on difficult missions for the Chaos Army, such as infiltrating enemy camps that are otherwise impenetrable.
Well, perhaps the plot could involve Percy getting tired of his normal life and being drawn to the chaos realm. There, he meets Zoe and they have some adventures together that make them fall in love. And finally, they decide to get married, facing any challenges that come their way in the chaotic world.
Obviously Percy is one of the main characters. There might also be the leaders of the chaos army. They could be some powerful and mysterious figures that Percy has to interact with. And perhaps some of Percy's old friends might also play a role, either trying to bring him back or having to face him as an enemy now.
In this kind of fanfiction, Percy might use his demigod powers in the military. For example, his ability to control water could be useful in various military operations like amphibious assaults. He could also use his combat skills learned from Camp Half - Blood to outperform regular soldiers.
If Percy Jackson joins Chaos in fanfiction, it would likely change his entire perspective. He's used to the world of Greek gods and demigods. But in the realm of Chaos, there are no such clear - cut rules. He could be sent on wild adventures across different dimensions. He might have to face enemies that are far more powerful and unpredictable than anything he's encountered before. And his relationships with his friends from Camp Half - Blood would also be put to the test as he becomes something more than just a demigod.
Well, one possibility could be that Hermione uses her vast knowledge to improve the army's strategies. Maybe she comes up with new spells or ways to defend against dark magic threats that the regular military tactics don't cover.
Well, it could be that Harry joins the army because he feels a need for a new purpose in life. The story might show him adapting to the strict military discipline. Maybe he'll be part of a special unit and be involved in some sort of mission to save the world in a different way than he did before. For example, instead of using magic directly, he might use his strategic thinking and leadership skills he's developed over time.
Maybe Bella has some personal growth. She could start as a timid person and the army experience makes her brave. For example, she overcomes her fear of heights during a training exercise.
One possible reason could be that his friends wrongly accused him of something major. For example, if they thought he was secretly working for an enemy and didn't give him a chance to explain. Another reason might be that the gods themselves turned against him unjustly, perhaps because they feared his growing power and influence. This could make Percy feel so alone and betrayed that he decides to join Chaos as a form of revenge or to find a new place where he is accepted for who he is.
If Percy Jackson is leading the Chaos Army in fanfiction, one thing that could occur is that he forms alliances with other powerful forces. Given his reputation and the fact that he has friends in many different places, he could convince other groups to join his cause. He might also face challenges from within the army, as some members may not be used to his leadership style. However, Percy would likely overcome these challenges by proving himself through his actions and by showing that his way of leading is the best for the overall success of the Chaos Army.