If Percy Jackson was raised in the Underworld, his personality would likely be much darker. The environment of the Underworld is gloomy and full of death. He could be more brooding and less trusting. His fighting style might also change, incorporating more of the Underworld's magic and powers. He may even have a different weapon, something more associated with the Underworld like a bone - made sword or a whip made from the shadows of the dead.
The plot of the story would take a whole new turn. Percy would likely be involved in different quests. Instead of quests related to saving the world from Titans or Giants, he could be involved in Underworld - related problems like stopping a rebellion among the dead or dealing with power struggles within the Underworld itself. His allies would also be different. He might team up with Underworld creatures like the Furies or the ghosts of powerful heroes instead of the demigods from Camp Half - Blood.
His character would become more confident in his artistic abilities. Apollo is the god of many arts, so Percy would likely be exposed to music, poetry, and painting. He might be more flamboyant in his actions and speech, as Apollo has a certain charm. For example, he could start using more flowery language when talking to his friends or enemies.
Percy's character would likely become more in tune with nature. He'd be more reserved perhaps, as he grows up in an all - female group. His sense of honor might be more shaped by Artemis' values, such as protecting the weak and the natural world.
The plot could be centered around Percy's identity crisis. Since he's raised by Apollo, he might struggle with whether he should follow Apollo's more flamboyant and carefree ways or stay true to his own nature. He could be exposed to Apollo's many lovers and children, which would make him question his place in this complex family. Additionally, his relationship with other demigods would change. He might be seen as an outcast by some who are loyal to Poseidon, while others might be jealous of his connection to Apollo. This would lead to various adventures where he has to prove himself not just as a son of Poseidon but as someone with a unique upbringing under Apollo.
One possible plot could be that Percy, being raised by Artemis, grows up with a deep connection to nature and the moon. He might be trained in archery and hunting from a young age. Since Artemis is a virgin goddess, Percy would have to deal with the unique situation of being part of her all - female group, perhaps facing some initial resistance from the hunters. He could also develop special powers related to the moon or gain an enhanced sense of stealth.
One possible plot could be that Percy discovers he has a special connection with dragons. Maybe he finds out that the dragons sense his demigod powers and take him in as one of their own. He then learns dragon - related skills like flying on their backs or using dragon fire in battles against his usual foes in the Percy Jackson universe.
The fanfiction could start with Percy being lost at sea as a baby. Instead of being found by his normal family, he is taken in by a pod of dolphins or some other intelligent sea beings. As he grows, he learns to communicate with all sea life. He then discovers his demigod heritage but feels more at home in the sea. His adventures could include using his sea - based powers to fight against underwater villains who want to disrupt the balance of the ocean world, and he has to decide whether to stay in the sea or embrace his role on land.
A new and powerful demon that has emerged from the darkest corners of the underworld. This demon is bent on using Percy's connection to the overworld to break through the barriers between the two worlds and unleash chaos. It has powers that are different from what Percy has faced before, like the ability to manipulate the very essence of the underworld's environment.