A third plotline could be about Percy's mental and emotional state during his banishment. He could be dealing with feelings of betrayal, as he was banished. He might question his identity and purpose, and throughout the fanfiction, he gradually overcomes these doubts and insecurities, emerging stronger in character even if he's still trapped in Tartarus.
Perhaps a prophecy misinterpretation. If a prophecy was misread and it seemed like Percy was the cause of great evil to come, the gods might take drastic action. For instance, if the prophecy spoke of a 'child of the sea' causing destruction, and they wrongly assumed it was Percy without fully understanding the context.
One possible plotline could be Percy's struggle for survival in Tartarus. He might have to face off against various terrifying monsters. Another could be his journey to find a way out while dealing with the psychological trauma of being in such a hellish place. And perhaps he discovers some long - hidden secrets about Tartarus itself during his time there.
The monsters native to Tartarus could also be antagonists. These could be huge, grotesque creatures that have never seen the outside world. They might attack Percy on sight, seeing him as prey. Some of them could be powerful enough to even give Percy a hard time, especially if his powers are weakened in Tartarus.
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.
One possible plotline could be Percy's struggle for survival in Tartarus. He might face various terrifying monsters and have to use his wits and powers to stay alive. Another could be his journey of self - discovery in that dark place, realizing new aspects of his abilities or his past. Maybe he also tries to find a way to escape Tartarus while also dealing with the psychological toll of being in such a desolate and dangerous realm.
One possible plotline could be Percy's journey of dealing with the diagnosis. He might initially be in denial, then gradually accept it and start treatments. Another could be how his friends and family react, like his friends rallying around him to keep his spirits up.
One possible plotline could be Percy's struggle to survive on his own outside of Olympus. He might have to hide from other mythological beings who are either after him or just not friendly to a banished Olympian. Another plotline could involve Percy trying to find a way to get back to Olympus, facing various challenges and making new allies along the way.
Well, one possible plot could be that Percy and Zoe have to face various ancient and terrifying monsters in Tartarus. They might encounter challenges that test their friendship and their individual strengths. For example, they could come across the Titans that were once banished there, and have to find a way to avoid or defeat them while also trying to find a way back to the upper world.
Well, perhaps in the fanfic, there's a prophecy that gets misinterpreted. The gods think Percy is going to cause great harm based on this wrong interpretation, so they send him to Tartarus. It could also be that there's some sort of power struggle among the gods. Percy might be caught in the middle of it, and one side decides to get rid of him by banishing him to Tartarus to tip the balance in their favor.