Mentally, it could be a very tough experience. He would be alone with his thoughts, which might lead to self - doubt or a sense of hopelessness. Since he's used to having friends to rely on, being alone could break his spirit. Also, without his friends' different skills and powers, he'd have to rely solely on his own abilities to survive and find a way out of Tartarus.
Another consequence could be that he discovers new aspects of his powers. The extreme environment of Tartarus might force him to dig deeper into his demigod abilities. Maybe he'll learn to control water in ways he never thought possible before, or develop some new combat skills in order to fend off the various threats that lurk in the dark abyss.
He might face extreme physical challenges like battling powerful monsters on his own. In Tartarus, there are many dangerous creatures that could be a huge threat.
One possible consequence could be that he becomes even more powerful in a dark way. Tartarus is full of dangerous and ancient forces. Staying there might expose him to unique and powerful magics that change him. For example, he could gain new combat skills or the ability to control certain Tartarus - born entities.
He might face numerous terrifying monsters. Tartarus is full of dangerous creatures, and without his friends, Percy would have to rely solely on his own skills and the power of his demigod abilities to fight them off.
His relationships with his friends could become stronger. When he eventually gets out, his friends might see him as even more of a hero for surviving Tartarus alone. They could respect him more and he might be more confident in his relationships.
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.
He uses his wits. Percy has always been clever, and in Tartarus, he'd use that to his advantage. He'd find ways to outsmart the monsters, like using the environment against them.
There are several possible reasons. Firstly, for the sake of drama. It makes for a very intense and emotional story when a beloved character like Percy dies. Secondly, it could be to explore the after - effects on the other characters. How would Annabeth cope? How would the Camp Half - Blood react? Also, it might be a way to show the unforgiving nature of Tartarus.
One consequence could be that the fanfiction community might feel less connected to the character. If Percy doesn't seem to interact with or acknowledge the fan - created stories, fans might feel like their creativity isn't being recognized. For example, they might be less motivated to write more stories about him.
It could lead to a breakdown in the demigod community. Percy is kind of a hero figure, and if he doesn't care, it might demoralize other demigods. They might start to question their own purpose and the importance of their roles. Also, it could make Camp Half - Blood more vulnerable to attacks as Percy is a strong fighter.
A third plotline could be Percy's search for a powerful artifact or knowledge in Tartarus that could help him in his future battles or save his friends. Along the way, he discovers secrets about the origin of the gods or the true nature of Tartarus itself, which changes his understanding of the world he thought he knew.
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.