Maybe an ancient and powerful sorcerer who has been awakened from a long slumber. He wants to use the powers of the demigods, especially Hylla and Percy, to regain his lost glory and rule the world. He uses dark magic and illusions to try and defeat 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.
One great 'Percy Jackson Hylla' fanfiction is 'Hylla's Journey'. It really delves into Hylla's character development in a world filled with the magic and chaos of Percy Jackson's universe. The author does an amazing job of showing her growth and how she interacts with the other characters from the series.
One idea could be that Hylla and Percy team up to rescue a group of demigods captured by an ancient monster. They use their combined powers and wits to outsmart the beast.
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.
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.
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.
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.
It could be an ancient sea god who has been forgotten but now wants to reclaim his former glory. He sees Percy's ocean soul as a key to achieving that and will stop at nothing to get it from him, causing chaos in the process.
It could be a new demigod who is jealous of Percy's newfound super speed and wants to take it for himself. He becomes the main obstacle for Percy throughout the fanfic.
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 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.