In some fanfictions, Percy might be more withdrawn. He has seen the horrors of Tartarus and it could make him less trusting of the world around him. He may also have new powers or a deeper understanding of his abilities that he gained from the experiences there.
Well, in a lot of fanfiction, Percy after Tartarus is often depicted as being more haunted. He could have nightmares regularly. His relationships with others might change too. For example, he may be more protective of Annabeth because of what they went through together in Tartarus. Also, his view of the gods could be even more jaded as he's seen the true depths of their realm in Tartarus.
In fanfiction, Percy can be really different after Tartarus. He might have a darker edge to his personality. The things he witnessed and endured in Tartarus could have toughened him up in a way that makes him seem colder on the outside. His fighting style could also be different. Maybe he's more brutal or efficient, having learned from the battles in Tartarus. His connection to the ocean could also be altered, perhaps stronger in some ways or more unpredictable due to the trauma he's been through.
Often, their relationship is shown as deeper and more unbreakable. They've been through Tartarus together, which is like the ultimate test. So in fanfiction, they trust each other completely and are always there for one another.
Well, they would probably communicate better. In Tartarus, they had to rely on clear communication to survive. So, back in the normal world, this would carry over. They'd be more open about their feelings, fears, and plans, which would strengthen their relationship in a more emotional way.
It can also lead to new character development arcs for other characters. If Percy is different, those around him have to adapt. Annabeth might have to learn how to deal with this new Percy, which could be a whole new plotline in the fanfiction. Maybe she tries to bring back the old Percy, or she has to fall in love with this new version of him all over again.
Percy could become more introspective. He's seen the worst, so he might start to question his role as a demigod more deeply. Annabeth, on the other hand, might be more assertive. For example, she could be more forceful in her plans to protect the camp. In fanfiction, these developments can make their relationship more complex and interesting.
In some fanfictions, Annabeth and Percy might struggle with PTSD. They could have nightmares about their time in Tartarus. Percy might be overly protective of Annabeth, and they may need time to heal their emotional wounds together.
His powers would likely be much darker. He might have the ability to manipulate shadows like Nyx, using them to hide or even attack. And from Tartarus, he could have the power to cause earthquakes or open up chasms in the earth. He could also have a natural resistance to the harmful effects of the Underworld, such as the poisonous gases and the draining of energy that normal demigods experience.
In some fanfictions, Percy might use his water - controlling abilities in unique ways. He could manipulate the rivers of Tartarus, like the Phlegethon, to create distractions or even use the power of the water to propel himself out. Maybe he also forms alliances with some minor creatures in Tartarus that are not as hostile as the main threats and they help him find a hidden passage.
In some fanfictions, Percy might gain power in Tartarus through facing and overcoming numerous deadly challenges. For example, by defeating powerful monsters that inhabit Tartarus, he could absorb some of their essence or gain knowledge from them which then transforms into power. Another way could be through making pacts with certain entities in Tartarus in exchange for power, although this would likely come with a price.
In some fanfictions, Percy might get hurt in Tartarus due to the numerous and powerful monsters there. For example, he could be injured by the Empousai. Their sharp claws and ability to move quickly can catch Percy off - guard, leading to cuts and bruises.
Surviving in Tartarus would require them to adapt quickly. The place is full of unpredictable and extreme conditions. They need to find shelter from the various hazards, like the volcanic eruptions and the attacks of powerful monsters. Their friendship and mutual trust would also be a great strength. They can support each other emotionally, which is crucial in such a terrifying and desolate place. For example, when one of them is feeling down or hopeless, the other can cheer them up and keep their spirits high, so they can keep on fighting for survival.