A group of rogue demigods could be the antagonists. They are jealous of Percy and Jason's fame and power. They form a secret alliance and start causing trouble for the other demigods. Percy and Jason meet while trying to figure out who is behind these attacks. At first, they suspect each other, but then they realize the real culprits are this group of rogue demigods and join forces to stop them.
A great antagonist could be a rogue demigod who has been corrupted by power. This demigod could have discovered a way to combine the darkest parts of Greek and Roman magic and is using it to cause chaos among both camps. He or she might see Jason and Percy as the biggest threats to their plans and target them specifically.
It could be an old enemy from their past, like a resurrected Titan or Giant. These powerful foes might manipulate Percy and Jason into betraying their friends as part of a grand plan to regain power or take revenge on the demigods.
In a 'Jason meets Percy' fanfic, they could meet in a place that is neutral to both of them, like a magical marketplace. Percy is there looking for a special item for his next adventure, and Jason is on a mission to find a way to strengthen his camp. They cross paths and initially have a bit of a misunderstanding. But as they start talking, they find out they have a common enemy in the background planning something big. So they decide to join forces and use their unique skills to stop the enemy.
Obviously, the gods themselves would be the main antagonists. Since they are the ones who kill Percy and Jason in this concept. It could be a group of gods, like Zeus, Poseidon, and Hera, who feel that the two heroes are getting too out of hand.
Perhaps they argue over a girl. They are both heroes and might have feelings for the same person. Jealousy and competition could spark an argument between them. Also, their loyalty to different camps, Camp Half - Blood and Camp Jupiter, could be a source of conflict. They might have different ideas on how to handle the relationship between the two camps, which would lead to arguments.
At first, it might show the tension between them due to Jason's jealousy. Jason could be distant or snippy with Percy. But as the story progresses, they might start to communicate. Maybe Percy notices Jason's jealousy and tries to reach out, which begins to thaw the ice between them.
One great example could be a story where Jason and Percy team up to fight a new and powerful monster. In this fanfiction, their different powers from their respective godly parents blend in an interesting way. For instance, Percy's control over water and Jason's ability to control the winds work together to create a powerful storm against the enemy.
Well, in this kind of fanfiction, Jason might start showing his jealousy in various ways. Maybe he'll be snippy with Percy when they interact. For example, he could make sarcastic remarks about Percy's abilities or how he's so popular. He might also try to one - up Percy in front of others, like during a training session or a quest briefing.
It could be an ancient sea creature that was once imprisoned by Amphitrite and Poseidon. Now that Percy has come into the picture, it senses an opportunity to break free. It uses its dark powers to manipulate the ocean and turn it against Percy and Amphitrite, hoping to create enough chaos to shatter the chains that bind it once again.
Perhaps a rogue group of mermaids and mermen. They don't like the idea of Triton allying with Percy, a half - blood. They believe that Triton should only focus on the pure - blooded sea creatures. So they become the main antagonists, constantly attacking Percy and trying to turn Triton against him.