Well, in most fanfictions where Percy trains in Tartarus, it often involves him facing extreme challenges. He might be learning to control new powers or just surviving the harsh environment. For example, he could be training to fight stronger monsters that lurk in Tartarus.
The plot might also include Percy's internal struggle. Training in Tartarus is no easy feat. He could be dealing with the psychological toll of being in such a dark and dangerous place. Alongside physical training, he has to overcome his fears and doubts. This could lead to some really interesting character development as he emerges from Tartarus a stronger and more confident hero.
Obviously, Percy would be one of the main characters. Since it's about him training in Tartarus. And Annabeth might also be involved. Maybe she's helping him in some way, or she's waiting for him outside Tartarus while he trains.
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.
Well, in many fanfictions, Percy might be on a mission to assassinate some powerful being in Tartarus. Maybe he has to face all kinds of hellish creatures and overcome insane challenges. It could involve him using his water powers in unique ways, like creating weapons out of water to fight the demons of Tartarus.
Well, perhaps Percy could be learning some new and powerful combat skills from Chaos. Maybe Chaos teaches him to manipulate chaos energy in unique ways for battle.
One possible plot could be Percy learning unique survival skills while training in Alaska. Maybe he has to deal with extreme cold, wild animals like bears or wolves. He could also meet local Alaskan characters who teach him their traditional ways of living and hunting, which in turn enhance his own training.
Perhaps the fanfiction could be about Naruto training Percy in stealth. Percy, being a demigod, often charges into battle head - on. Naruto, on the other hand, is a master of stealth. He could teach Percy how to move quietly and undetected. This would be a great advantage for Percy when he has to infiltrate a monster's lair or spy on the enemy. The story could show Percy struggling at first but gradually getting better with Naruto's guidance.
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.