One possible plotline could be Percy having nightmares about his time in Tartarus. He might struggle with PTSD, which could affect his relationships with his friends and family at Camp Half - Blood. Another could be that he brings back some sort of dark power or knowledge from Tartarus that he has to figure out how to control or use for good. Maybe he discovers a way to close the entrance to Tartarus permanently so that no more monsters can come out and endanger the demigod world.
Well, he could return with some new abilities. For example, he might have developed an enhanced sense of danger, like being able to sense when a monster is approaching from Tartarus even before it arrives in the mortal world. His return could also trigger a series of events where old enemies from Tartarus start to target him specifically. And he would have to rely on his friends, like Annabeth and Grover, to help him fight off these new threats while also dealing with the internal changes he has gone through.
Perhaps Percy returns with a newfound sense of purpose. In the fanfiction, he could be on a quest to find a way to purify the area around Tartarus to prevent the evil that seeps out from corrupting the world. He might also face suspicion from some of the other demigods who are afraid that he has been tainted by Tartarus. His journey could involve him proving himself once again, this time not just as a hero, but as someone who has overcome the darkest place in the demigod realm.
Some of the more powerful and vengeful spirits in Tartarus could be the main antagonists. These spirits could possess demigods or mortals close to Percy to get to him. They might be angry that he escaped their domain and want to drag him back. Additionally, there could be a group of rogue demigods who believe that Percy has been corrupted by Tartarus and they see it as their duty to stop him, even if it means becoming his enemies.
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.
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.
Well, it could involve Percy coming back with new powers. Maybe he's more in tune with the underworld's magic. He might have made some sort of deal in Tartarus and now has to deal with the consequences in the mortal world.
He might face mental challenges like PTSD. Tartarus is a horrible place, and the things he witnessed there could haunt him.
He could come back haunted by the terrors he faced in Tartarus. In the fanfiction, Percy might struggle with nightmares and flashbacks. His relationship with Annabeth could be strained as he tries to deal with his trauma alone. But then, through the help of his friends and the love of Annabeth, he slowly starts to heal. Meanwhile, an old enemy from Tartarus could be following him back to the surface world, and Percy has to gather his courage again to fight it off and protect his loved ones.
One possible plotline could be Percy being framed for a crime he didn't commit within the demigod world. This would lead to his fall from grace as his friends and allies start to doubt him. Maybe he's accused of stealing a powerful artifact that is crucial for the safety of Camp Half - Blood. As a result, he has to go on the run, trying to clear his name while also facing the challenges of being alone and hunted.
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.
Well, one possibility is that he has to find a way to survive on his own. He might encounter new and even more terrifying monsters in Tartarus. Maybe he'll discover some long - lost secrets about the place that could be used to his advantage or that could change the whole understanding of the demigod world if he ever gets out.