Maybe it's not a single god but a group of minor gods who feel overshadowed by the major gods' attention towards Percy. They band together and spread false accusations against Percy, leading to his torture in front of the gods. They hope to regain some of the power and respect they think they've lost due to Percy's presence.
The main antagonist could be an ancient and powerful god who has been sleeping for a long time and wakes up to find Percy as a symbol of the new order of demigods that he doesn't like. This god could use his influence over the other gods to have Percy tortured in front of them all to assert his dominance and stop the rise of the demigods.
It could be Ares. He's always been sort of a hot - headed and war - loving god. Maybe he's jealous of Percy's popularity among the demigods and wants to see him suffer. So he manipulates the other gods into believing Percy has done something wrong and that he should be tortured.
Chiron could also be a key figure in trying to save Percy. As the centaur trainer at Camp Half - Blood, he has a lot of knowledge and connections. He might try to negotiate with the gods on Percy's behalf or find some loophole in the gods' actions to free Percy.
In this fanfiction, it might be that the gods are influenced by an external dark force. This force convinces them that Percy is a threat. So, instead of a fair trial, they torture him in front of everyone. Percy, being a hero, would probably endure it with great courage and his friends might be on a quest to prove his innocence and save him from this unjust situation.
Maybe the main antagonist is an old enemy of Percy's who has learned about his newfound ability to hypnotize (even if it was accidental). This enemy decides to use Annabeth as a pawn by hypnotizing her, hoping to lure Percy into a trap and defeat him once and for all.
The main antagonist might be a new character, a powerful being that rules the realm where Percy is banished. This being could see Percy as a threat to his or her power and try to stop Percy from finding a way back.
The gods themselves could be the main antagonists. Since they cursed Percy, they might be trying to keep him in a state of misery or use him for their own purposes.
It could be an ancient and powerful god or goddess who has a grudge against Percy. Maybe they see Percy as a threat to their power or plans, so they plot his death. For example, a forgotten deity who wants to regain their former glory by getting rid of Percy, who is often seen as a hero in the Percy Jackson universe.
It could be an envious demigod who feels that Percy doesn't deserve his new position as a consort of the gods. This demigod might plot against Percy, trying to undermine his influence among the gods and the other demigods.
A group of rogue demigods who have turned against the gods and see Percy as a traitor for being so close to them. They form an alliance and try to stop Percy from fulfilling any quests or tasks given to him by the gods, and might even try to capture or kill him.
Set, the Egyptian god of chaos. He could be a great antagonist as he might see the combination of Greek and Egyptian powers in Percy and the other demigods as a threat to his own plans of spreading chaos. He could try to manipulate events to turn the two groups against each other.
The main antagonist could be one of the major gods who cursed Percy. For example, Zeus might be angry with Percy for some reason and decide to curse him. Zeus would then use his power to send all kinds of obstacles and enemies Percy's way. He could manipulate the weather to create storms that target Percy or send powerful monsters after him.