In this fanfiction, Percy could also become a sort of bridge between different worlds. His human experiences combined with his new family in the realm of Nyx and Erebus could allow him to bring about changes. He might be able to use his knowledge of both worlds to resolve conflicts or create new alliances. For instance, he could help the Olympians understand the primordial forces better, or vice versa.
His life would be very different. He'd likely be living in a much darker and more mysterious environment. The powers he'd be exposed to would be more primordial in nature. He might have to deal with the shadows and secrets that come with being part of Nyx and Erebus' world.
The theme of identity is also quite prevalent. Being the son of such powerful and somewhat mysterious entities, Percy might struggle with his own identity. Is he more like his father, Erebus, associated with darkness and shadow, or his mother, Nyx, the night? Fanfictions often play with this idea, showing his journey to understand who he is and where he fits in the grand scheme of the mythological universe.
One common theme is the struggle with identity. Since Percy is the son of Erebus and Nyx in these fanfictions, he often has to grapple with being different from the 'canon' Percy. Another theme is power. Given his new parentage, he might have different or enhanced powers, and the fanfictions often explore how he uses or controls them.
Perhaps a really interesting plot twist could be that Percy's adoption by Erebus reveals a long - lost prophecy. This prophecy states that Percy, with his combined heritage of being Poseidon's son and Erebus' adopted son, is the key to either the destruction or the salvation of the entire Greek mythological world. This would put Percy in a very difficult position as he tries to figure out how to fulfill the prophecy in the best way possible, all while dealing with the reactions of other gods, demigods, and mythical creatures.
The Titans could be antagonists. Since Nyx is an ancient and powerful goddess, the Titans might see Percy as a threat because of his connection to her. They could try to either turn him against Nyx or simply eliminate him.
He would probably become an even more formidable figure. With Nyx's blessing, he could have enhanced senses during the night, making him a better fighter in the dark. And from Uranus, he might receive a boost in his leadership abilities. Since Uranus was an important god in the Greek pantheon, this could give Percy more influence among the demigods and gods alike. He could be seen as a special agent of change, with the combined powers of these two deities.
Another plot could involve Percy being drawn into a quest related to the primordial world of his new parents. Maybe there's an ancient threat that only someone with his unique heritage can stop. This would take him to places like the Underworld in a whole new light, as he has a deeper connection to the primordial darkness there.
In the 'Percy is adopted by a Norse god' fanfiction, he could be given a new identity within the Norse world. This new identity might come with certain responsibilities. He may have to prove himself to the Norse community. And his friends from the Greek side might have different reactions. Some could be jealous, while others might be curious about his new life. There could also be a plot where Percy has to use his combined Greek and Norse knowledge to save both worlds from a common threat.
Well, Percy might gain new powers related to the Underworld. Since Hades is the god of the Underworld, he could teach Percy how to control shadows or communicate with the dead. This would make Percy an even more formidable demigod, with abilities that no one else has. He could also face a lot of challenges from other gods who might be jealous or scared of his new connection to Hades.
In this fanfiction, Percy could be exposed to different realms and dimensions that are part of Chaos' domain. He might meet some very strange and powerful creatures. Maybe he'll form alliances with them or learn from them to become an even greater hero. Also, his personality could change as he adapts to this new and wild environment under Chaos' care.