Maybe it's part of a plot to have Percy gain some sort of power or knowledge. In many fictional stories, a character has to go through great pain and suffering before emerging stronger or wiser. The torture in Tartarus could be a way to set up this transformation for Percy.
Percy might endure it through his own stubborn will. He's a demigod used to facing tough situations. In this fanfiction, he could be using his past experiences of fighting monsters and surviving hardships. He might tell himself that he's been through worse and this is just another trial he has to pass. So he grits his teeth and bears it as best he can, even when the torture in Tartarus is beyond what he's ever faced before.
Another scenario could be that Percy is forced to endure extreme environmental conditions in Tartarus. The place is known for its intense heat, toxic fumes, and jagged terrain. He might be made to traverse through burning lava fields or climb sheer cliffs while being constantly weakened by the poisonous air, all of which would be a form of torture.
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.
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.
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.
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.
There's no way to tell who the author is just from the title 'percy is tartarus fanfiction'. It could be written by any fan of the Percy Jackson series who wanted to explore this particular scenario in their own writing. It might be someone new to fanfiction writing or an experienced writer in the Percy Jackson fan community.
Well, it could be about Percy's experiences in Tartarus. Maybe it's a fictional account of Percy facing the extreme challenges and horrors that Tartarus has to offer. It could involve his battles, his relationships with other characters while in that hellish place, and how he overcomes the difficulties there.
Perhaps he could make an alliance with some of the less hostile creatures in Tartarus. There might be beings who are also tired of the chaos and want to help Percy in exchange for something, like a promise to free them from Tartarus if he ever gets the chance. Percy is known for his charisma and ability to make friends, even in the most unlikely situations.