One challenge could be dealing with the trauma. Tartarus is a terrifying place, and Percy might have nightmares that disrupt his school life. He could also find it hard to relate to his classmates who have no idea what he's been through.
Another aspect is his relationships. Some of his friends might have changed during his absence. There could be new social dynamics at school that he has to figure out. Also, he may be more withdrawn due to his experiences, which could strain his friendships.
Percy might face academic challenges. After such a harrowing experience, it could be difficult for him to focus on schoolwork. His mind might be preoccupied with the memories of Tartarus, and he may have fallen behind in his studies while he was away.
He might face mental challenges like PTSD. Tartarus is a horrible place, and the things he witnessed there could haunt him.
One challenge could be readjusting to the village life. After intense training in a different environment, the daily routines and social norms of Konoha might seem strange. Also, he may face jealousy from some of his peers who haven't advanced as much in their skills during his absence. Another challenge could be dealing with the new threats that Konoha might be facing, and having to prove himself all over again to the village elders that his new skills are useful for the village's protection.
The environment itself is a big obstacle. Tartarus is a place of extreme darkness and chaos. It can mess with Percy's mind, making it difficult for him to focus on his training. The terrain is also treacherous, with rivers of fire and bottomless pits.
One of the main challenges would be the dangerous environment. Tartarus is filled with all kinds of monsters and hazards. Percy would constantly have to protect the child from these threats. Also, the lack of proper resources like food and clean water could be a huge issue. There's not much in Tartarus that is suitable for a growing child.
One major challenge could be the powerful and malevolent creatures that inhabit Tartarus. Monsters like the Titans or other ancient evils would try to stop him. They are much stronger than the usual foes he faces on the surface. Another challenge is the very nature of Tartarus itself. The place is designed to be inescapable, with its dark magic and chaotic environment constantly working against Percy.
One challenge could be the language difference. Even though English is spoken in both places, the accents and some local slang in England might be confusing for Percy. For example, he might struggle to understand what people mean when they use terms like 'bloke' or 'cheeky'.
One possible plot could be Percy dealing with PTSD from Tartarus while trying to fit into high school life. He might have nightmares that disrupt his school days and struggle to make friends who can understand his past traumas.
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 major challenge could be trust issues. Since the world of espionage is full of double - crossers, Percy might constantly be wondering who he can really trust within the agency and outside. Another challenge could be the physical danger. He could be sent on dangerous missions where his life is at stake, like infiltrating enemy strongholds.
Perhaps there's a new villain, a powerful sorcerer who has a vendetta against Percy. The sorcerer discovers that Percy is now at high school and decides to target the school. He uses his magic to create illusions and manipulate the minds of the students and faculty, turning them against Percy. The sorcerer might also be after the secrets Percy learned in Tartarus, thinking it can give him more power.