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.
In this 'Percy becomes dark fanfiction', it might be that the main antagonist is an ancient evil that has been awakened. This evil entity preys on Percy's insecurities and the challenges he faces. It manipulates the events around him, causing him to question his values and eventually turn dark. The entity could be some sort of demon or an all - powerful being from a forgotten era.
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.
It could be the previous Dark One who doesn't want to give up their power completely. Even though Emma has become the new Dark One, the old one might try to manipulate her from the shadows, trying to get her to make mistakes that would allow them to regain their power.
Maybe a group of rogue demigods who have formed a sort of 'dark gamer' cult. They believe that by dominating the gaming world, they can gain enough power to overthrow the Olympian gods and rule over all. They see Percy as a threat to their plans and constantly try to thwart him in the games and in real - life situations.
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.
The main antagonist could be a cult that worships the dark god of lust. They might be trying to bring about the full power of the dark god by using Percy as a sacrifice or a vessel. They would be constantly opposing Percy and his friends, trying to stop them from thwarting their evil plans.
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.