Maybe there's a group of rogue demigods who are jealous of Percy's new status as Artemis' husband. They try to create problems for them, like spreading false rumors or trying to steal their powers. These demigods could be led by someone with a grudge against either Percy or Artemis.
Another antagonist could be a dark force from the Underworld. Since Percy has a connection to the water and Artemis is a goddess, their combined power might be seen as a threat to the balance of power in the Underworld. So Hades, or some other dark entity under his command, could try to break them apart or stop them from achieving their goals as a married couple.
One common antagonist could be an old enemy of Artemis who doesn't approve of her marrying Percy. This enemy might be a powerful Titan who sees their union as a threat to his plans for revenge against the gods.
It could be some rogue Titans who escaped from Tartarus. They want to cause chaos in Oklahoma and use Percy and Artemis to their advantage. Maybe they plan to capture them and use their powers to break free the other Titans.
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.
The Titans could still be antagonists. Even though Percy is now associated with Artemis, the Titans might see him as a threat or a tool to use against the gods. Kronos, for example, could try to turn Percy to his side by offering him power or by threatening those he cares about in Artemis's group.
In this fanfiction, the Titans could be the main antagonists. They might see Percy as a tool to use against Artemis. Since he's her son, they could try to manipulate him or turn him against her. Another possible antagonist could be the monsters that are drawn to Percy because of his unique heritage. They might sense his power and want to either destroy him or use him for their own evil purposes. Additionally, there could be a group of rogue demigods who are jealous of Percy's connection to Artemis and try to undermine him at every turn.
One possible antagonist could be an old enemy of Percy's who has a vendetta against his son. For example, a powerful Titan who was once defeated by Percy might see the silver son as a way to get revenge. They could try to corrupt the son or use him against Percy and his friends.
The other gods might be antagonists. For example, Zeus could be against Artemis getting Percy back because he doesn't trust Percy or he has his own plans for Percy that don't align with Artemis'. Another god like Ares could also be an antagonist as he might see Percy as a threat and not want him to be on Artemis' side.
The main antagonists could be some old enemies of Percy or Reyna who don't want to see them happy. For example, a vengeful monster from Percy's past might try to disrupt their relationship. Maybe it's a Titan who was once defeated by Percy and wants to ruin his newfound happiness with Reyna.
Some ancient and powerful monsters could be antagonists. Since Percy is now connected to the Fates, any disruption in the natural order might attract these monsters. They could see the situation as a chance to gain power or revenge. For instance, a powerful Titan who was once imprisoned might see Percy's new status as a way to get back at the gods by attacking him.
The antagonists could be some rogue scientists who are experimenting on demigods like Percy. They might be trying to harness the powers of the demigods for their own gain, whether it's for power, wealth, or some other self - serving reason.
A strict teacher could be an antagonist. Maybe this teacher has old - fashioned views and doesn't like Percy's unorthodox ways or Artemis's independent streak. The teacher could constantly give them detention or try to stop them from participating in certain school events that are important for the plot.