Maybe it could be a rogue demigod who is jealous of Percy. This demigod could be working with some dark forces and tries to make Percy's situation worse while he's holding up the sky. They could spread rumors among the demigod community to turn others against Percy, making it harder for his friends to get help.
It's possible that the main antagonist is an ancient and malevolent spirit that has been trapped for ages and sees Percy's predicament as an opportunity. This spirit could be influencing the minds of some of the demigods and creating obstacles for Percy's friends. It might also be able to directly attack Percy while he's holding up the sky, trying to make him drop it.
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.
The main antagonist could be a powerful sorcerer who has found a way to enter the Percy Jackson game world. He has his own motives, perhaps he wants to steal the powers of the demigods for himself. He's able to manipulate the game's code to his advantage, creating illusions and false paths for Percy and his friends to follow. Or it could be an ancient spirit that was sealed away in the game but has been released. This spirit is angry at the gods and demigods and wants to destroy everything they hold dear. Also, a group of bandit demigods could be the main antagonists. They don't follow the rules of the game or the real - world demigod society and are causing chaos everywhere they go, trying to steal valuable items from both the game world and the real - world locations.
Nyx could be a great main antagonist. She's a primordial goddess of the night and Tartarus is her domain in a way. She might see Percy and his friends as intruders and try to stop them from leaving or achieving their goals. She could use her power over darkness and the unknown to create all sorts of obstacles for them.
It could be a new, powerful Titan that has emerged from the depths of Tartarus. This Titan might be seeking revenge on the demigods for imprisoning the other Titans and sees Hylla and Percy as the biggest threats.
A powerful sorcerer from another realm who uses assassins as tools. This sorcerer could see Percy as a threat to their plans of taking over the demigod world. They send their best assassins after Percy and also try to manipulate other demigods against him. They are extremely cunning and have a vast array of dark magic at their disposal.
The main antagonist could be a powerful Titan who wants to use Percy as a weapon to overthrow the Olympian gods. The Titan might see Percy's unique powers as a weapon as the perfect tool to gain ultimate power and reshape the world according to his own desires.
Even some of the ancient demigods who turned bad could be antagonists. They might see Percy as a threat to their power or plans in the past. They could have different values and goals compared to Percy, leading to conflicts. For example, a demigod who was power - hungry and wanted to use time - travel to change things for his own gain, and Percy would have to stop him.
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.
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.