A powerful mythological creature that has a grudge against the god Percy is championing for. For example, a Titan who was once defeated by that god and now sees Percy as a way to get revenge. It would be a great adversary, with its own set of powers and minions.
Maybe a group of mortals who have been corrupted by dark magic. They see Percy as a threat to their new - found power and plan to stop him. They could use modern technology combined with dark magic to create unique challenges for Percy, making them an unexpected and formidable antagonist.
It could be one of Mars' old enemies, like a powerful Titan who has a grudge against Mars and sees Percy as a threat. Maybe this Titan wants to take over Mars and Percy stands in the way.
It could be an old enemy who now sees Percy as an even greater threat. Maybe Kronos' remnants. Since Percy is now a god, they might fear that he'll be able to fully destroy them. So they try to stop him from getting used to his powers or try to steal his powers.
A powerful Titan who wants to use Percy's newfound power as the God of Pain for his own evil purposes could be the main antagonist. This Titan might see Percy as a tool to spread chaos and destruction, and he tries to manipulate Percy into using his pain - based powers to harm innocents. Percy would then have to resist this Titan's influence and fight against him.
Perhaps a forgotten or banished god could be the main antagonist. This god, filled with resentment for being cast out, could return with a vengeance and start causing chaos. He might be powerful enough that the only solution Percy sees is to slay him. This antagonist could bring in a lot of new lore and backstory to the fanfic as Percy uncovers the secrets of this god's past.
Perhaps it could be a new evil force created as a result of Percy's sudden god - born existence. His new - found power could have unbalanced some ancient magic or prophecy, and this new evil entity rises to oppose him and restore the old balance. It might be a being made of dark magic that has a vendetta against Percy for disrupting the natural order of things.
A powerful sorcerer who once was a disciple of Hecate but turned to the dark side could be the main antagonist. He might be jealous of Percy being chosen as the champion and wants to steal Percy's powers for himself.
It could be an old enemy of Percy who has somehow gained power over the Styx. Maybe a resurrected villain from Percy's past who now wants to use the power of the Styx to take revenge on Percy and the gods. For example, Kronos' remnants could have found a way to manipulate the Styx's energy for their own evil purposes.
A powerful Titan who has somehow escaped Tartarus and is bent on revenge against the gods could be a great antagonist. This Titan might see Percy as a key figure in the new order of the gods and fates, and so wants to destroy him. Maybe it's someone like Atlas, who has a long - standing grudge against the Olympians and now wants to take it out on Percy as the champion of the fates.
There could be a new Titan that we haven't seen before. This Titan could be more cunning and powerful than the others. Maybe he's been in hiding all this time, waiting for the right moment to emerge and lead the Titans to victory. He could manipulate Percy's friends against him and create all sorts of problems for our hero.
It could be a powerful demigod who has been corrupted by power. This demigod wants to be the sole champion of the arena and will stop at nothing to defeat Percy.