It could be a new monster that is specifically targeting Percy's mortal connections, like Paul. Maybe a monster that feeds on the fear of mortals who are close to demigods.
There's a possibility that it could be a rival demigod who is jealous of Percy's relationship with mortals, especially Paul. This rival demigod might see Paul as a weakness in Percy and try to use him to get to Percy. They could create all sorts of problems for Paul and Percy, like setting up traps or spreading false rumors about Percy among the mortal community that Paul is part of. The rival demigod could also try to drive a wedge between Percy and his friends at camp by using Paul as a scapegoat.
It could be a new monster that is after Percy. Since Percy is a demigod, there are always monsters chasing him. This new monster might be something that Paul has never seen before and it could put both of them in danger.
Well, Paul might be really shocked at first. Percy could show him some of his demigod powers, like controlling water. Paul, being a normal mortal, would be super amazed and probably a bit scared too.
In this fanfiction, perhaps Paul Blofis, being Sally's husband, could offer Percy some fatherly advice. He might share his own experiences and wisdom with Percy, which could help Percy in dealing with his own complex life as a demigod. Percy, on the other hand, could introduce Paul to the magical aspects of his world, like the existence of Camp Half - Blood and the different gods.
Maybe a group of mercenaries who are also interested in the valuable artifacts related to Greek mythology. They would be willing to do anything to get their hands on them, and they are a threat not only because of their weapons but also because they have some knowledge of the ancient secrets that could be used against Percy and Lara. They could set up ambushes and try to steal the artifacts from Percy and Lara while they are in the middle of their exploration.
Paul might be characterized as the voice of reason. In a world full of chaos and mythical beings, he can be the one who tries to find logical solutions. He could be a bit of an amateur detective in the fanfiction, using his teaching skills to research and figure out things related to the demigod world. Also, his relationship with Sally gives him a deeper connection to Percy and a motivation to be involved in the demigod's life, so his protective side towards Percy can be emphasized.
He could interact with them in a very friendly and awestruck way at first. Like he's constantly asking questions about their powers and adventures, and the Avengers are amused by his enthusiasm. For example, he might be star - struck when he meets Iron Man and keeps asking about his latest suit upgrades.
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.
Another possible antagonist could be a powerful sorcerer or witch who is envious of Aphrodite's beauty and power. They see Percy as a way to get to Aphrodite. So they try to use Percy as a pawn, casting spells on him to make him do their bidding and cause harm to Aphrodite, all while Percy is trying to figure out what's going on and break free from the control.
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.
There could be a new, made - up character. A digital wizard of sorts who has learned how to cross over from the popularmmos world to the Percy Jackson world. He wants to control all magic and technology, believing that by merging the two worlds under his rule, he can become the most powerful being. So he sets out to stop anyone who stands in his way, including Percy and the popularmmos crew.