Perhaps a jealous sea nymph. She has always been envious of Amphitrite's position and power. When she sees Amphitrite bonding with Percy, she decides to be the antagonist. She spreads false rumors about Percy among the sea creatures, causing some to be wary of him and Amphitrite's association with him. She might also try to sabotage any attempts at further bonding between them, like ruining a special ceremony or event they were planning to attend together.
A plot for 'Percy and Amphitrite bonding fanfiction' could be that Amphitrite notices Percy's unique relationship with the sea. She decides to take him under her wing to teach him the proper ways of the sea god's court. Percy is initially intimidated but soon realizes that Amphitrite is not as strict as he thought. They bond during lessons on sea - based powers and how to control the tides. As their relationship grows, Percy becomes more respected in the underwater world, and Amphitrite gains a new perspective on the half - bloods through Percy.
It could be an ancient sea creature that was once imprisoned by Amphitrite and Poseidon. Now that Percy has come into the picture, it senses an opportunity to break free. It uses its dark powers to manipulate the ocean and turn it against Percy and Amphitrite, hoping to create enough chaos to shatter the chains that bind it once again.
Some of the traditional sea monsters like the Kraken could be antagonists. Since Percy is now in a more prominent position in the sea world as Amphitrite's adopted son, the Kraken might see him as a threat and try to attack him.
One possible plot could be Percy discovering his true parentage as the son of Amphitrite. He might struggle with his identity, as he has always known Poseidon as his father. This could lead to a journey of self - discovery where he has to learn about Amphitrite's powers and how they are now a part of him.
In such a fanfiction, Percy could have a very different personality. Since Amphitrite is a goddess, she might raise Percy with a more formal and disciplined upbringing compared to his normal life. He could be more in tune with the politics and hierarchies of the underwater world, and perhaps have a more regal bearing. He might also have a special relationship with sea creatures that are more closely associated with Amphitrite, like certain types of sea nymphs or powerful sea monsters.
It could be a new character, like a powerful wizard who has a grudge against Percy for some reason. Maybe Percy accidentally disrupted one of his spells, and the wizard is out to get him, which makes Percy mad.
It could be a new god or goddess who is jealous of the attention Percy gets. Maybe they try to disrupt the conversation between Athena and Percy to cause chaos.
It could be the warden of the prison. The warden might be a cruel and power - hungry person who wants to keep Percy imprisoned for his own reasons, like getting some sort of reward or power from those who put Percy there.
A powerful dark sorcerer could be the main antagonist. He might be seeking to use Percy's skills for his own evil purposes, and when Percy refuses, he becomes the target. The sorcerer would have a vast array of dark magic at his disposal, making him a formidable foe.
It could be an old enemy of Percy's, like a vengeful god or a powerful monster. They might be targeting Percy's guardian to get back at Percy.