I'm not entirely sure as it could be about many things. It might be a fan - created story where Percy is portrayed as the son of Chaos. Maybe it involves Percy having special powers or going on unique adventures because of his relation to Chaos.
You could try looking on popular fanfic websites like Archive of Our Own (AO3) or FanFiction.net. They have a large collection of fan - created works and there's a good chance you'll find it there.
One interesting plotline could be Percy's journey in the realm of Chaos. He might face new and extremely powerful enemies that test his skills in ways not seen in the original series. For example, he could encounter primordial entities that are far more ancient and mysterious than the Titans or the Giants.
Percy's friends from Camp Half - Blood could also be important. They might react in different ways to Percy's new status as the Son of Chaos. Grover, for instance, could be torn between his loyalty to Percy and his fear of the new and unknown powers Percy wields. Nico might also have a significant role, perhaps using his own dark powers to help Percy understand and control his Chaos - related abilities. These characters add depth to the story and create a sense of continuity with the original Percy Jackson series.
One possible plot could be Percy's struggle to balance his two powerful heritages. He might face challenges from both the forces of Chaos and the underwater world of Poseidon. For example, the denizens of the deep might be wary of his Chaos side, seeing it as a threat to their ordered underwater society. Meanwhile, the chaotic entities could try to pull him fully into their realm, away from his connection to Poseidon.
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.
His powers could develop in a very unique way. Since he has the blood of Chaos, his powers might start off as small, random bursts of energy. For example, he might accidentally create small pockets of chaotic space. As he learns to control his Poseidon powers, like controlling water, he could start to merge the two. He might be able to make the water act in chaotic ways, like creating whirlpools that have unpredictable movements.
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.