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.
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 great antagonist could be a rogue demigod who has been corrupted by power. This demigod could have discovered a way to combine the darkest parts of Greek and Roman magic and is using it to cause chaos among both camps. He or she might see Jason and Percy as the biggest threats to their plans and target them specifically.
It could be an old enemy of Percy and Annabeth. Maybe a monster that they thought they had defeated long ago but has come back for revenge. This time, it might target Annabeth's daughter to get back at them.
There could be a new, made - up character. A digital wizard of sorts who has learned how to cross over from the popularmmos world to the Percy Jackson world. He wants to control all magic and technology, believing that by merging the two worlds under his rule, he can become the most powerful being. So he sets out to stop anyone who stands in his way, including Percy and the popularmmos crew.
Perhaps there is no traditional antagonist. Instead, the conflict could be within Percy himself. His future self might have some dark secrets or regrets that Percy in the present doesn't want to face. So, in a sense, Percy's own denial or fear could be the 'antagonist' in this fanfiction.
Perhaps a rogue group of mermaids and mermen. They don't like the idea of Triton allying with Percy, a half - blood. They believe that Triton should only focus on the pure - blooded sea creatures. So they become the main antagonists, constantly attacking Percy and trying to turn Triton against him.
Another possible antagonist could be a powerful sorcerer or witch who is envious of Aphrodite's beauty and power. They see Percy as a way to get to Aphrodite. So they try to use Percy as a pawn, casting spells on him to make him do their bidding and cause harm to Aphrodite, all while Percy is trying to figure out what's going on and break free from the control.
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.
It could be a new monster that is after Percy. Since Percy is a demigod, there are always monsters chasing him. This new monster might be something that Paul has never seen before and it could put both of them in danger.
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.