In such a fanfiction, Percy might use his knowledge of the future to change some key events in the past. For example, he could prevent some of the earlier disasters or conflicts in the Percy Jackson universe. Maybe he'd try to stop the first encounter with a certain monster in a different way to avoid the chaos it caused.
One possibility is that Percy is sent back to the time when the gods were more involved in mortal affairs. He could have a chance to directly interact with them in a more profound way. He might try to convince the gods to be more lenient with demigods or to change some of the rules that make demigods' lives so difficult. This could lead to a whole new series of adventures and relationships within the fanfiction world.
Perhaps Percy will discover secrets about his family or his powers that were previously unknown. Since he's in the past, there could be ancient magic or knowledge that he stumbles upon. This newfound knowledge could give him an edge when he returns to his own time, if he ever does. He might also face challenges like trying to blend in with the people of the past, as his modern ways could make him stand out and cause suspicion.
He could potentially change some events in the past that had a big impact on his present life. Say, he could prevent some of the early attacks by monsters that made his life so difficult growing up. This would completely alter the course of his life and the lives of those around him. It's a really cool concept to think about how one small change in the past could have a huge ripple effect in the future.
He could meet his parents when they were young. Maybe he'd get to see his mother before she met his father and understand more about her life back then.
He could meet his younger self. This would be a really interesting encounter as he could either try to guide his younger self or be shocked at how different he was back then. Another possibility is meeting some of the ancient heroes that he only heard about before. For instance, he might come across Hercules and learn from him.
In fanfiction, if Percy Jackson is sent to Tartarus, he might face countless deadly monsters. Tartarus is a really dangerous place. He would have to use all his skills and powers to survive. Maybe he'd meet some old foes there, and it would be a great test of his courage and determination.
Well, Percy could end up changing the entire course of history in the 'Percy is thrown back in time' fanfiction. He might accidentally stop some important relationships from forming or start new ones that were never meant to be. Also, he could have a chance to see his loved ones who are no longer alive in his original time, which would be an emotional rollercoaster for him.
One interesting plot idea could be Percy going back in time to the era of the Greek heroes. He could meet Hercules and learn from him. Maybe he would have to face the same challenges Hercules did, but with his own modern - day skills and knowledge. Another idea is that he goes back to the time when the gods first emerged and gets involved in the power struggles among them. This could change the entire course of mythology as we know it.
Another possibility is that his time - traveling could create some sort of time paradox. This might lead to all sorts of strange and unexpected consequences. His presence in the past could disrupt the normal flow of magic and time, and he'd have to find a way to fix it before everything goes wrong.
If Percy Jackson is sent to Tartarus in fanfiction, it could be a harrowing experience. First off, the environment itself is a nightmare. The terrain is dangerous and full of pits and lava. Percy would have to rely on his wits as much as his strength. Since Tartarus is the prison for the most powerful Titans and monsters, he'd be up against foes far more powerful than he's used to. He might even discover some long - lost secrets about his powers or his family's past while trying to find a way out.
It could be an ancient and powerful Titan who was thought to be defeated long ago. Since Percy is in the past, this Titan could be in his prime and see Percy as a threat to his plans of ruling over the world again. For example, Kronos might be even more powerful in this past era and Percy has to find a way to stop him without the help of his modern - day friends.