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.
Perhaps the main antagonist could be a primordial being that has been awakened by the changes in the Styx due to Percy's new role. This being, long forgotten by the gods, sees Percy as an obstacle to its plan of reclaiming the world. It would have vast and terrifying powers, and Percy would have to use all his skills, along with the powers of the Styx, to stand a chance against it.
The Titans could still be a major threat. Even if Lady Styx is involved, the Titans might see an opportunity to use the chaos she causes to their advantage. For example, Kronos could try to ally with Lady Styx to regain his power and overthrow the Olympians once again.
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.
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.
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.
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.
Well, perhaps Percy in the fanfiction is faced with a moral dilemma related to his role as Champion of Styx. For example, he might be asked to make a sacrifice that goes against his principles but is necessary to save the world. Or he could be dealing with the jealousy and suspicion of his friends who don't fully understand his new powers and status. This could lead to some really interesting character development and story arcs.
Another antagonist could be a powerful frost giant. If Percy is involved in protecting Asgardian interests related to Thor, a frost giant could be a major foe. They are strong and have their own motives for opposing Thor and his champion. They might see Percy as an easy target at first but soon realize his strength.
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.
Well, clearly Percy Jackson is one of the main characters. He's a well - known demigod in the Percy Jackson universe. And of course, Lady Styx is another main character in this fanfiction. She has her own powers and influence in the Underworld.
Some of the gods might also be important characters. Poseidon, Percy's father, could have an interesting reaction to Percy's new relationship. Hades, as the god of the underworld where Styx is located, might also be involved, either as an ally or an obstacle depending on the direction of the fanfiction.