Well, it could be that Percy is tricked into believing the son of chaos is a threat when he's actually not. Maybe some other characters manipulate Percy's perception with false evidence.
It could be an ancient evil god who manipulates Percy into betraying the son of chaos. This god might have a long - standing grudge against the son of chaos or his lineage and sees Percy as a useful pawn.
Perhaps in the fanfiction, Percy is influenced by some dark magic or a curse that makes him betray the son of chaos. The son of chaos then has to decide whether to forgive Percy or seek revenge. The story could also explore how their friends and allies are affected by this betrayal and whether they side with Percy or the son of chaos.
It could be the gods. They often have their own agendas and might see the son of chaos as a threat to their power. So they might manipulate Percy into betraying the son of chaos.
In some fanfictions, Percy Jackson is indeed portrayed as the true son of Chaos. Writers often take creative liberties to explore new storylines and character developments. This idea can lead to very interesting and unique plot twists, such as Percy having powers beyond what we see in the original series.
After joining Chaos, Percy's character evolves into a more complex one. He is no longer the naive hero he once was. He gains a new sense of purpose. With Chaos, he might be involved in more dangerous and morally ambiguous situations, which forces him to re - evaluate his values. He may become more independent and self - reliant, relying less on the support he used to get from his old friends and more on his own skills and the new allies he finds in Chaos.
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.
The plot could center around Percy's transformation into the 'Son of Chaos'. He might have to face challenges that test his loyalty to his old friends at Camp Half - Blood while also adapting to his new role. He may also encounter new characters who are either allies or enemies in this new phase of his life. It might also involve a power struggle within the chaotic realm where he has to prove himself worthy.
One possibility for a Percy Jackson fanfiction centered around the Primordial Son of Chaos could be a story where Percy discovers a hidden prophecy related to this powerful being. Maybe he has to team up with some unlikely allies, like a group of minor gods who are also interested in the balance that the Primordial Son of Chaos might disrupt or uphold. This could lead to a series of adventures across different realms, including Tartarus, where they search for clues about the true nature of the Primordial Son of Chaos.