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 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 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.
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.
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.
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 interesting plot could be Percy's struggle to survive in Tartarus alone. He might have to face the various monsters there without his usual allies. Another plot could involve him finding a way to escape Tartarus using his wits and the powers he has. Maybe he discovers some secrets about Tartarus that no one else knows.
Obviously, Percy would be the main character. Since he is the one sent to Tartarus. Annabeth might also play an important role, even if she's not in Tartarus with him at first. She could be on the surface world trying to figure out a way to get to him or send help. Grover could be another likely character. His connection to nature might come in handy in some way, like if there are strange nature - based elements in Tartarus that only he can understand or control.
Well, a plotline could be Percy having to adapt to the extreme and harsh environment of Tartarus. The place is full of darkness and danger. He might have to learn to harness the strange and dark energy that exists there in order to defend himself. There could also be a plot where Percy is forced to make difficult choices. For instance, he might be presented with an option that could save him but at a great cost to others. This would test his moral compass and show how much he has grown as a character in this extreme situation.
Obviously, Percy is the central character. But often, Annabeth can also play an important role, even if she's not physically in Tartarus with Percy in some stories. She might be trying to find a way to rescue him from the outside. Another common character is Nico, who has some knowledge of the Underworld and Tartarus and could potentially be a key figure in helping Percy either survive or escape.