The plot could be centered around Percy's struggle to reintegrate into his normal life after Tartarus. He could be haunted by the nightmares of the place. His relationships with Annabeth and his other friends might be strained as he tries to hide the full extent of what he endured. On the other hand, he might also be on a mission to prevent something from Tartarus from escaping into the world, using the knowledge he gained during his time there.
In this fanfiction, perhaps Percy returns with a deeper understanding of the dark forces. He could bring back knowledge of ancient and dangerous creatures from Tartarus. His friends might be both amazed and a little scared of the changes in him. He may also face challenges as others in the demigod world are suspicious of his time in Tartarus and what he might have become.
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.
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.
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.
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 plot could be Percy and Zoe facing numerous dangerous monsters in Tartarus. They might have to rely on each other's unique abilities. For example, Percy's control over water could help them cross certain deadly rivers while Zoe's hunting skills could be used to fend off the approaching beasts.
One possibility could be Percy's struggle to survive the harsh conditions. He might have to find new ways to use his powers in Tartarus. For example, the strange and dark magic there could either enhance or limit his abilities. He could also form alliances with unexpected creatures. Maybe he teams up with some of the ancient and terrifying monsters that live there, but they have a common enemy, so they work together for a time.
Well, one possibility could be Percy facing his inner demons. In Tartarus alone, he might have to deal with the memories of his past failures and the fear that comes with being in such a desolate place. He could encounter strange and terrifying creatures that test his combat skills and his will to survive.
One possible plot could be that Percy returns with new powers. Maybe he was resurrected by some ancient magic or a powerful deity who has a plan for him. He then has to deal with the shock and awe of his friends and also face new enemies who are either threatened by his return or want to use his newfound powers for their own gain.
Obviously Percy himself. Annabeth would likely be there as she has a close relationship with Percy. Grover might also show up, being Percy's long - time friend. And some of the other demigods from Camp Half - Blood, like Jason or Piper, could be in the story too.
Perhaps Percy has to go to Tartarus alone because he believes it's the only way to save his friends. In Tartarus, he might encounter old foes who are now even more powerful. He has to rely on his wits and his skills with his sword, Riptide. He could also make unexpected alliances with some of the creatures there in order to survive and achieve his goal.
In this fanfiction, Percy might also have to deal with his own sanity. Growing up in Tartarus, the constant threat and the dark, desolate place could be a huge mental strain. He could struggle with nightmares and hallucinations, and part of the plot could be about him overcoming these mental challenges while trying to find a way back to the world above.