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.
He might use his sword skills and his ability to control water. For example, he could create shields out of water to protect himself from attacks. He would also try to find any weaknesses in his enemies. If there are any water sources in Tartarus, he could use them to his advantage, like making geysers to distract or attack his foes.
He might rely on his powers. Percy is a son of Poseidon, so his control over water could be crucial. He could use water sources in Tartarus, if any, to heal himself, create barriers, or even find a way to escape. For example, he could manipulate the Styx water to his advantage.
Percy might feel a deep sense of hurt. He values friendship highly, and this betrayal would make him question who he can trust. He could become more guarded and less likely to trust new people easily. Reyna would be more focused on the bigger picture. She would see the betrayal as a threat not just to herself but to the stability of the demigod world. So she would start gathering her allies, those who still believe in her, and form a strategy to counter the false accusations and the forces behind the betrayal. She would be determined to prove that she and Percy are not the traitors they are made out to be.
Perhaps Percy would try to understand the primordial's reasons for the betrayal. He's a smart and empathetic character. He might think that there was some misunderstanding or that the primordial was forced into it. So, instead of immediately retaliating, he could try to communicate with the primordial to clear things up. If that doesn't work, then he would be more likely to take a defensive stance rather than an offensive one, at least until he fully understands the situation.
Nico di Angelo could be involved. Since he has some knowledge of the Underworld and Tartarus, he could play a crucial role. He might be the one to guide Annabeth or other rescuers to the right places in Tartarus to find Percy. Also, some of the Titans or primordial deities who have a connection to Tartarus could be important characters, either as obstacles or potential allies for Percy.
Percy might be angry at first. He's used to being a loyal friend, so betrayal would hit him hard. He'd probably try to find out the truth and clear his name. Jason, being a more diplomatic type, might initially be shocked and then try to reason with those who betrayed him. Nico, on the other hand, could retreat into himself. He has a more solitary nature, and the betrayal might make him even more withdrawn.
Percy might initially be confused. He would probably wonder why Annabeth, who was once his friend or ally, is treating him this way. So he could try to talk to her to find out what's wrong.
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.
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.