One possible reason could be jealousy among his so - called friends. Maybe someone was envious of Percy's powers or popularity and decided to frame him, leading to his betrayal and subsequent sending to Tartarus. Another reason might be a misunderstanding. For example, if there was some false evidence planted against Percy, his comrades could have been misled into believing he had done something unforgivable and sent him to that horrible place.
Perhaps there was a power struggle within the group Percy was a part of. Some individuals who wanted more power saw Percy as an obstacle. They spread lies about him and convinced the others that he was a danger. As for being sent to Tartarus, it could be that the group had a false belief that Tartarus was the only place where Percy's 'evil' (which was actually fabricated) could be contained.
Percy could initially be heartbroken. He's a very loyal person, and to be betrayed by those he considered family would be devastating. In Tartarus, he would have to rely on his instincts and his powers. He might also experience a lot of self - doubt, wondering if he really was as bad as they made him out to be. As time passes, he would start to plan his revenge against those who wronged him while also looking for an escape route.
Maybe it's part of a plot to have Percy gain some sort of power or knowledge. In many fictional stories, a character has to go through great pain and suffering before emerging stronger or wiser. The torture in Tartarus could be a way to set up this transformation for Percy.
One possible storyline could be that Percy is tricked into betraying Tartarus by some dark force. Maybe they use his friends as hostages or manipulate his mind. Another could be that Percy believes he's doing the right thing in betraying Tartarus, but later realizes it was a huge mistake and has to face the consequences. For example, he might face the wrath of Tartarus' minions or lose the trust of his allies.
There are several possible reasons. Firstly, for the sake of drama. It makes for a very intense and emotional story when a beloved character like Percy dies. Secondly, it could be to explore the after - effects on the other characters. How would Annabeth cope? How would the Camp Half - Blood react? Also, it might be a way to show the unforgiving nature of Tartarus.
The plot could revolve around Percy's journey of self - discovery in Tartarus. Sent there unjustly by the gods, he could start to question his past actions and his relationship with the divine. As he battles the horrors of Tartarus, he may develop new powers or skills. He could also be on a mission to expose the true nature of the gods' actions, which led to his banishment to Tartarus. Along the way, he would face challenges that test his physical and mental strength to the limit.
Perhaps a prophecy misinterpretation. If a prophecy was misread and it seemed like Percy was the cause of great evil to come, the gods might take drastic action. For instance, if the prophecy spoke of a 'child of the sea' causing destruction, and they wrongly assumed it was Percy without fully understanding the context.
Annabeth would be a likely character to appear. Since she and Percy are so close, she might try to find a way to rescue him from Tartarus. Another character could be Nico. He has experience with the Underworld and might be able to offer some help or advice. Grover could also show up, using his nature - related powers to try and sense Percy in Tartarus.
One possible plotline could be Percy's struggle for survival in Tartarus. He might face various terrifying monsters and have to use his wits and powers to stay alive. Another could be his journey of self - discovery in that dark place, realizing new aspects of his abilities or his past. Maybe he also tries to find a way to escape Tartarus while also dealing with the psychological toll of being in such a desolate and dangerous realm.
Obviously, Percy is one of the main characters. Since he's the one sent to Tartarus. There might also be some of his old friends who play important roles, like Annabeth. Maybe she's on a quest to save him, so she becomes a central figure in the fanfiction.
It could be another powerful god or goddess who wants to use Percy's betrayal of Tartarus for their own gain. Maybe a god of chaos who sees this as an opportunity to create more disorder in the world. They might manipulate Percy into the betrayal and then turn on him later.