In this fanfiction, a key plot point could be Percy forming unexpected alliances. Maybe he teams up with some of the less - evil creatures in Tartarus to help him in his quest. Also, his internal struggle with fear and despair would be an interesting aspect. He's used to being a hero, but Tartarus might test his limits and make him question his abilities.
A great plot could involve Percy finding a way to use the unique environment of Tartarus to his advantage. For example, there could be some sort of energy in Tartarus that he discovers he can harness to become stronger, but it has a dangerous side - like it might slowly corrupt him. He then has to struggle with the decision of whether to keep using it to get out or resist it.
They could discover a lost civilization in Tartarus. This civilization might have a way to manipulate the very fabric of Tartarus itself. Percy and Annabeth would then have to learn the ways of this civilization in order to use it to their advantage. Maybe they have to earn the trust of the civilization's leaders by helping them with their own internal problems first.
One idea could be Percy having to prove himself in the Underworld. Since he's the son of Tartarus, he might face unique challenges from the other denizens who fear or distrust him. Maybe he has to compete in some sort of underworldly tournament to gain respect.
Obviously, Percy would be the main character. He's the one stuck in Tartarus, so the story would revolve around him. Maybe Annabeth could also be an important character, even if she's not physically in Tartarus with him. Her thoughts and attempts to rescue Percy from afar could be part of the story.
Another plot could be that something from Tartarus has followed Percy out. A small, malevolent creature that starts causing trouble in Camp Half - Blood. Percy feels responsible for it and has to find a way to send it back to Tartarus without putting everyone in danger.
One possible plot twist could be that Percy discovers a hidden power within Tartarus that he can use to his advantage. Maybe it's a long - lost magic that only the darkest part of the Underworld can unlock.
Another plot twist might be that Tartarus itself has a weakness that Percy discovers by accident. For example, he might notice that certain sounds or vibrations can disrupt the structure of Tartarus. This would completely change the game as he could then use this knowledge to his advantage, perhaps causing mini - earthquakes to distract or even harm the monsters that are after him.
In some fanfictions, a possible plot development is that Percy and Annabeth discover a long - lost secret or power within Tartarus. This could be something that gives them an edge in their fight to escape or a knowledge that changes their perspective on the whole Greek mythology world. As they explore Tartarus, they may find ancient artifacts or meet forgotten gods who have been trapped there for ages. These encounters can lead to unexpected alliances and revelations that add depth to the story.
Well, perhaps a plot twist is that Percy doesn't escape in the traditional sense. Instead, he finds a way to manipulate the rules of Tartarus so that he can exist in both Tartarus and the mortal world simultaneously. This could lead to some really interesting story developments as he tries to keep this a secret from his friends while also using his new - found ability to fight against the forces of evil. Also, it could be that the escape route he finds is actually a trap set by an enemy, but he manages to turn it around and use it to his advantage in a really unexpected way.
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.