It could be an ancient and powerful god who has long been plotting against Percy. This god might be jealous of Percy's power and popularity, so he devises a plan to turn Percy into a killer, knowing that it will lead to Percy's downfall and his own rise in power.
Maybe it's a new and mysterious character. This character could be from a different dimension or realm and has a vendetta against Percy's world. He uses dark magic to influence Percy and make him a killer. The character might have some deep - seated grudge against the entire concept of heroes, and Percy is just the first step in his plan to destroy all that is good.
It could be a character who wants to use Percy - turned - Aku for their own evil purposes. Maybe a dark wizard who thinks that Aku - Percy can be the ultimate weapon to take over the world. This wizard would try to manipulate Percy's new form and powers to do his bidding.
It could be a fallen angel who is jealous of Percy's new - found angelic status and wants to bring him down.
It could be a powerful leader from another race who sees Percy - the newly turned Zerg - as a threat to their own plans for domination. Maybe a human general who once knew Percy and now wants to destroy him because he has joined the Zerg side.
It could be a character who is against the idea of Percy being a girl. Maybe someone who believes in strict gender roles in their world and sees Percy's transformation as an abomination. For example, an old - fashioned wizard or a powerful deity.
There could be a group of poachers who are not just after regular game but are also involved in some dark magic. They oppose the hunters and especially Percy when he tries to stop their illegal and dangerous activities. They might have powerful backers or magic items that make them a formidable foe for Percy and the other hunters.
Perhaps it's one of Percy's former friends who has turned against him. This friend could be jealous of Percy's powers or has been influenced by some dark force. Their actions and taunts could be what drives Percy to become dark, making them the main antagonist in the story.
Perhaps Percy is pushed to his limits by some great injustice in the story world. Maybe he witnesses the death of someone he loves at the hands of an evil force. This trauma could transform his personality, leading him to become a killer with a vendetta. He might start by targeting the ones directly responsible and then expand his scope as his thirst for vengeance grows.
A possible main antagonist could be Eris, the goddess of strife. She might be jealous of Aphrodite's beauty and power and see Percy as a tool to disrupt Aphrodite's plans. Eris could send her minions to create chaos and conflict between Percy and Aphrodite, trying to make them turn against each other.
It could be Morgana. She might see Merlin's transformation into a dragon as a threat to her own power and plans. So, she tries to stop him from fully becoming a dragon or tries to use his new form for her own evil purposes.
It could also be a new villain that is specifically interested in Sonic because she is now a girl. Maybe a villain who has some sort of vendetta against female heroes or wants to use Sonic's new form for some evil experiment. However, Dr. Eggman is so iconic in the Sonic series that he would be a strong contender for the main antagonist role even in this fanfic scenario.