Another option could be a group of rogue demigods. These demigods have turned against the Olympians and see Weaver as someone who could potentially foil their plans. They might have their own ideas about how the world of demigods should be run and are willing to do anything to achieve their goals, including getting rid of Weaver.
Some powerful and malevolent spirits from the Underworld could also serve as antagonists. They are envious of the living world and Weaver's abilities. They try to possess him or those around him to create chaos in the world above. They could use their powers of illusion and fear to try and drive Weaver mad or make him turn against his friends.
A group of rogue demigods could also work well as antagonists. They might be jealous of Percy and his brother's abilities or have their own twisted ideas about how the demigod world should be run. They could create all sorts of trouble for the brothers, from stealing their quests to setting traps for them.
Well, there could be some ice - related monsters or creatures that serve Khione. For example, ice giants or frost - breathing serpents. These could be constantly attacking Percy or trying to prevent him from achieving his goals while he's Khione's champion. Also, there might be some jealous demigods who think Percy is getting special treatment from Khione and they become antagonists by trying to sabotage him.
In Percy Jackson father fanfiction, the sea god Triton could be an antagonist. He might be jealous of Percy getting so much attention from Poseidon. The primordial sea gods or goddesses could also be antagonists. They might see Poseidon's relationship with a half - blood as a perversion of the natural order. And then there are always the minor gods who might feel that Percy and Poseidon are getting too much power and try to undermine them.
The Giants might also be antagonists. They could band together and target Percy's kingdom, believing that by destroying him and his rule, they can undermine the power of the gods further. They might use their brute strength and strategic thinking to attack Percy from different fronts.
Another option could be a group of rogue demigods who have turned to the dark side. They follow Percy to Forks, hoping to find some powerful artifacts there. They don't care about the vampires or werewolves and are willing to cause destruction to get what they want. Percy has to stop them while also dealing with the local supernatural creatures.
The spirits of the maze itself could turn against Percy. They could be angry at all the demigods who have been trespassing for years and decide to stop Percy from getting through by any means necessary.
Other powerful demigods could be antagonists. They might see Percy as a threat because of his primordial lineage and try to stop him from achieving his goals. Maybe some of them are being influenced by a malevolent god who doesn't want Percy to discover his full potential.
The Titans could be great antagonists. They might see the daughter of Hecate as a threat to their plans of taking over. Her magic could be something they want to either control or destroy.
The Titans could be the main antagonists. Even though they have been defeated before, some of them might have found a way to return or their followers could be causing trouble. They see Artemis as a threat or a target for their revenge against the gods. Another possibility is a new group of powerful monsters that have banded together and are specifically targeting Artemis for her power and influence among the demigods.
Well, the main antagonists could be a group of rogue demigods who have formed a cult around Kronos. They believe that Percy, as the heir of Kronos, should lead them in overthrowing the current order of the demigod world. But Percy doesn't want any part of it, so they become his enemies. Another possible antagonist could be an ancient being that was once in league with Kronos and now sees Percy as a means to resurrect Kronos or continue his dark legacy. This being would stop at nothing to force Percy to embrace his role as the heir.