A great antagonist could be a Titan who has a grudge against Zeus and sees Percy as an easy target to get back at him. For example, Atlas. He could try to use Percy to free himself from his burden again or trick Percy into doing something that would cause chaos among the gods. Another option is a powerful and vengeful demigod who feels that Percy, as the son of Zeus, has been given too much power and attention, so he wants to take Percy down.
It could be Hades in this fanfic. Hades might see Percy as a threat to his power, especially if Percy is the son of Zeus. He could try to stop Percy from achieving his goals, like preventing him from fulfilling a prophecy that would make Percy even more powerful. Hades could send his minions, like the Furies or powerful undead warriors, after Percy at every turn, creating constant danger for our hero.
Maybe it could be a new, made - up character. A powerful sorcerer who was once wronged by Zeus in the past and now wants to destroy anything related to him, including Percy. This sorcerer could have the ability to manipulate time and space, making it extremely difficult for Percy to fight against him. He could also create illusions to deceive Percy and lead him into dangerous situations. The sorcerer would be a formidable foe as he has spent centuries plotting his revenge.
It could be a powerful Titan who escaped from Tartarus and wants revenge on Zeus by targeting his son Percy. This Titan might have unique powers that make him a formidable foe.
A powerful Titan who sees Percy as a threat to his plans. Since Percy has the power of Zeus and Poseidon, he could potentially stop any chaos the Titan wants to create. The Titan might try to turn Percy's friends against him or find a way to strip him of his powers.
A powerful Titan who was once imprisoned by Zeus and now wants to destroy his legacy. This Titan could be seeking revenge for his long imprisonment and sees the legacy as a symbol of Zeus's power that he wants to crush.
The Titans could be the main antagonists. Even though they were defeated before, they might see Percy as a threat because of his powerful parentage. They could try to stop him from fulfilling a prophecy or gaining more power. For example, Kronos might have some remaining influence and try to manipulate events to destroy Percy.
The main antagonist could also be an ancient and powerful spirit from the underworld that was sealed away long ago. Percy's presence as the son of Hades accidentally awakens this spirit. The spirit then starts to spread darkness and despair throughout the world of the demigods, and Percy has to find a way to stop it before it's too late.
It could be one of Kronos' old allies who feels betrayed that Percy is now in a position of power. Maybe someone who was promised a lot by Kronos but didn't get it.
A new and powerful Titan who has been hiding and gathering strength all this time. He could be jealous of the Olympians' power and wants to take over. Percy would have to figure out his weaknesses and stop him before he causes too much destruction.
Hera could be a main antagonist. Since she's often jealous and protective of Zeus, she might not be happy with Percy being adopted. She could try to undermine him at every turn. Another could be Ares. He might see Percy as a threat, especially if Percy starts to gain more power as Zeus' adopted son.
Some of the minor gods who feel overshadowed by Zeus and Hera might be antagonists. In this fanfiction, they could see Percy as a symbol of the over - dominance of Zeus and Hera. They might plot against Percy to send a message to his parents. These minor gods could use their unique powers and their knowledge of the magical world to create all sorts of problems for Percy, from cursing him to setting up traps in his path.
A group of rogue sky spirits might serve as the main antagonists. They are jealous of Percy's status as the 'Son of the Sky' and want to steal his powers. They could be constantly causing chaos in the sky, like creating dangerous storms and attacking flying creatures. Percy would have to figure out their motives and stop them before they cause too much damage. These sky spirits could have their own hierarchy and leader, making them a formidable foe.