Perhaps Percy discovers a prophecy related to his birth. It could state that he is the key to either saving or destroying the world. This would create a lot of internal conflict for him. On one hand, he has his friends and the people he cares about in the mortal world. On the other hand, his powers and his role according to the prophecy could force him to make some very difficult choices.
Percy might initially be an outcast among the other demigods. His parentage is quite different from the normal Olympian - related demigods. They might be wary of him because they don't understand his powers. However, some brave demigods could be curious and try to befriend him.
A plot twist could be that Percy has a connection to the primordial forces that even the Titans and the Olympians are afraid of. His link to Nyx and Tartarus means he can access parts of the universe that are normally off - limits. For instance, he might be able to open portals to the darkest corners of Tartarus, which could either be a great advantage or a huge danger. Also, he could have the power to call upon the creatures of the night created by Nyx, and these creatures could be either his allies or a threat to the existing order of the demigod world, depending on how Percy chooses to use his powers.
The plot could involve Percy's journey to discover the true meaning of being the son of Gaea and Pontus. He could be on a quest to find out more about his parents' realms and how they interact with the world of the demigods. Along the way, he may encounter mythical creatures that are related to Gaea and Pontus, and he has to either fight them or make peace with them. There could also be a sub - plot about Percy's relationships with his friends at Camp Half - Blood, as they struggle to accept this new side of him.
Well, it could be about Percy having to deal with the powers and heritage that come with being the adopted son of Pontus. Maybe he discovers new underwater abilities that are different from his Poseidon - given powers. He might also have to face enemies who are after the secrets Pontus has.
One idea could be Percy discovering new powers related to being the son of Pontus. For example, he might be able to control underwater currents in ways he couldn't before. Another plot could involve him having to face a new type of sea monster that is specifically drawn to his Pontus heritage. He could also go on a quest to find an ancient artifact related to Pontus that has the power to either save or destroy the underwater world.
One interesting plot twist could be that the daughter of Nyx has a hidden power that is the key to saving Olympus from a new, unknown threat. Maybe she can control time in a way that no one expected, which throws the whole dynamic between the gods and demigods into chaos.
One idea could be Percy having to face a new type of monster that is related to Nyx's powers. For example, a shadow - creature that can only be defeated by using the unique abilities he inherits from being the son of Nyx.
Perhaps an ancient sea god who was once an enemy of Pontus. This god could see Percy as a threat because of his heritage. He would use his own minions, like sea serpents and murlocs, to attack Percy and those he cares about. He might also try to corrupt Percy, tempting him with power to turn against his friends.
Well, in fanfiction, if Percy is the Champion of Pontus, his story would be filled with more mystery. Pontus is an ancient and powerful entity. Percy might be called upon to do things that go against the traditional ways of the Olympian - influenced world he's used to. He could discover secrets about the origin of the sea and his own powers that were previously unknown. This would not only make for an exciting read but also show how Percy can adapt and make his own path even when given a very different role.
One idea could be Percy's struggle with his new identity as Nyx's adopted son. He might face discrimination from some of the other demigods who are suspicious of his new connection. Another plot could involve Percy learning new powers from Nyx that set him apart in battles against monsters. He could also have to deal with Nyx's enemies who target him because of his relation to her.