Who could be the main antagonist in 'percy grows up alone fanfiction'?

It could be a jealous demigod who sees Percy's wings as an unfair advantage. This character might try to find a way to cut off Percy's wings or steal the power that gave him the wings.

It could be Tartarus itself personified. As Percy is alone in this terrifying place, Tartarus could take on a form and directly oppose him. It would use all the horrors and challenges of the place to try and break Percy.

Well, one possibility is that Percy has to learn to fend for himself in a harsh world. Maybe he discovers unique powers within himself as he faces various challenges alone. For example, he could learn to hunt and build shelters all by himself, much like a survival story.

Well, for this type of fanfiction, the plot might start with Percy being separated from his usual allies very early on. He'd be forced to navigate the mortal world by himself. Since he's growing up alone, he may not have the same understanding of his Greek heritage as he does in the original series. This could lead to him making different choices when it comes to using his powers. For example, instead of using them to protect Camp Half - Blood, he might use them to simply survive in a world that seems to be constantly against him.

It could be Nyx. She is a very powerful primordial goddess in Tartarus. Since Percy is alone, she might see him as an easy target or a threat to her domain. Nyx could send all sorts of dark creatures and illusions to stop Percy from achieving whatever goal he has in Tartarus.

Percy's character would be very different. Growing up alone, he would lack the social skills he has in the original series. He'd be more introverted. His view of the gods would be more cynical as he has no one to tell him the 'glorified' stories. He'd have to learn about his powers through trial and error, which would make him more cautious. He may also develop a stronger survival instinct. His relationships would be different too. He might be more hesitant to open up to others when he finally does meet people. Overall, his character would be a loner with a unique perspective on the demigod world.

It could be a new character. Maybe a dark sorcerer who has come to Neverland while Peter was growing up. This sorcerer wants to take over Neverland and sees Peter, with his new adult understanding and power, as the only obstacle. The sorcerer could start to corrupt the island, turning the mermaids and Lost Boys against Peter, making him fight not only for his place in Neverland but also for the very soul of the island.

It could be a ghost hunter who is specifically targeting Danny because of his new fangs. This hunter might think that Danny's fangs are a sign of him becoming more dangerous and decides to take him down before he causes any real harm.

A new and powerful Titan could be the antagonist. One that has been hiding in the shadows and now sees Percy alone as an easy target. Maybe this Titan has a special grudge against Percy and wants to take him down while he's vulnerable.

It could be a new dark wizard who is more powerful and cunning than Voldemort. This wizard might have been in hiding all this time, biding his time to take over the wizarding world. With Harry being alone, he would pose a great threat as Harry doesn't have his friends to support him in this fight.