There's also the possibility that a new character could be Percy's savior. Maybe a powerful and mysterious demigod who has been hiding their powers all this time. This character could have some special connection to Tartarus or a unique ability that allows them to enter the realm without being immediately overpowered. They could be drawn to Percy's plight and decide to risk everything to save him.
Another possibility is that there are some good - hearted Shield agents who discover the truth about Percy. They could form an underground resistance within Shield. They might be people who have always been skeptical of the harsh treatment of Percy. They could then work together to free Percy and expose the wrongdoings of those in Shield who ordered the torture.
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.
His friends from Camp Half - Blood, like Annabeth, Grover, and Tyson, could be the potential saviors. They have always been by his side and would do anything to rescue him.
A plotline could be Percy having to face his own fears and insecurities while being tortured. Maybe he starts to doubt his abilities and heroism. But as the story progresses, he overcomes these doubts through his willpower. Also, the fanfic could explore how the torture affects his relationship with the gods. He might feel betrayed by them for allowing him to end up in Tartarus and this could lead to a change in his loyalty or attitude towards them.
He might use his powers in creative ways to lessen the impact of the torture. For instance, if the torture involves some form of mental attack, he could try to use his connection to water to calm his mind. Also, his demigod abilities, like his superhuman strength and agility, could help him physically resist some of the more brutal forms of torture to a certain extent. And he may also draw strength from the memories of his past victories and the people he has saved.
In some fanfictions, Percy's torture in Tartarus could be part of a grander scheme. For example, an enemy on the surface world has made a deal with the forces of Tartarus to weaken Percy. As he endures the torture, he gradually uncovers the conspiracy and tries to find a way to not only escape but also stop those who sent him there. His friends, unaware of his situation at first, start to suspect something is wrong and embark on a quest to find out what happened to him and rescue him.
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.
Grover could also be an ally. His connection to nature might give Percy some unique advantages in Tartarus. Additionally, some of the less hostile creatures in Tartarus, like the friendly Cyclopes, could side with Percy. They might see him as a more just ruler compared to the chaos that usually reigns in Tartarus. Also, Chiron, with his vast knowledge and experience, could offer guidance from afar, which could be considered a form of alliance.
It could be a group of rogue demigods who are jealous of Percy's status and power. They might think that by kidnapping and torturing him, they can gain some of his abilities or just to prove that they are stronger.
The main antagonist could be Nyx. She is a powerful primordial goddess of the night, and Tartarus is her domain in a way. She might see Percy as an intruder and try to stop him from escaping or achieving his goals there.