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.
His friends like Annabeth or Grover could be the ones to rescue him. They might discover a way into Tartarus and brave all the dangers to get to Percy.
Annabeth could be a likely candidate. She's always been by Percy's side, and she'd stop at nothing to save him. Her intelligence and combat skills would be useful in the dangerous environment of Tartarus.
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.
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.
Perhaps one of the Titans who were banished to Tartarus. They might see Percy as a threat to their plans of revenge or escape. Kronos, if he still has some form of existence in Tartarus, could be a major antagonist. He would have his own motives and powers, and he could try to manipulate Percy or simply get rid of him.
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.
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.
He might face terrifying monsters. Tartarus is full of the most dangerous creatures in the Percy Jackson universe. So he'd have to fight for his life constantly.
He could find some minor gods or goddesses who have been banished to Tartarus. They might see Percy as a way to get revenge on the Olympians or simply a means to escape. Another potential ally could be some of the more intelligent monsters, like a centaur who has been there for a long time and has knowledge of the place.
In some fanfics, Nico di Angelo could be the one to help Percy. Since Nico has a connection to the Underworld and has some dark powers, he might be able to enter Tartarus more easily than others. He could use his powers to fend off the monsters and guide Percy to safety.