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.
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.
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.
In fanfiction, if Percy Jackson is sent to Tartarus, he might face countless deadly monsters. Tartarus is a really dangerous place. He would have to use all his skills and powers to survive. Maybe he'd meet some old foes there, and it would be a great test of his courage and determination.
It's possible that an unexpected character like a friendly Titan could rescue Percy. There could be a Titan who has had a change of heart and realizes that Percy is important for the balance of the world. This Titan might use its own knowledge of Tartarus and its powers to free Percy from the clutches of that terrifying place.
One possible plot could be Percy having to face his inner demons in Tartarus. Maybe he has to battle new and terrifying monsters that are unique to that realm. He could also encounter old foes who have been made even more powerful in Tartarus.
One interesting idea could be Percy's struggle to survive in Tartarus. He might have to face new and terrifying monsters there. Another idea could be his journey of self - discovery in such a harsh environment. Maybe he'll find new powers or a deeper understanding of his own capabilities.
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.
He could be angry. Percy might lash out at those who betrayed him, feeling a deep sense of injustice. He would probably be determined to prove his innocence and get revenge on those who wronged him.
At first, Percy would likely be angry. He's a fighter, and being betrayed by someone he loves would fuel his rage. He might go after Annabeth to demand an explanation. But as the reality sinks in, he could become very sad and withdrawn. He might isolate himself from his other friends as well, feeling like he can't trust anyone anymore. He would constantly be thinking about what he did wrong to make Annabeth turn on him.