In some fanfics, Nico di Angelo could be the one to help Percy. Since Nico has a connection to the Underworld and has some dark powers, he might be able to enter Tartarus more easily than others. He could use his powers to fend off the monsters and guide Percy to safety.
He might face terrifying monsters. Tartarus is full of the most dangerous creatures in the Percy Jackson universe. So he'd have to fight for his life constantly.
He could find some minor gods or goddesses who have been banished to Tartarus. They might see Percy as a way to get revenge on the Olympians or simply a means to escape. Another potential ally could be some of the more intelligent monsters, like a centaur who has been there for a long time and has knowledge of the place.
One interesting plotline could be Percy's struggle against the various monsters in Tartarus. He might have to use his wits and powers in new ways to survive. Another could be his journey to find a way out, perhaps making unexpected alliances with other beings trapped there.
It's possible that an unexpected character like a friendly Titan could rescue Percy. There could be a Titan who has had a change of heart and realizes that Percy is important for the balance of the world. This Titan might use its own knowledge of Tartarus and its powers to free Percy from the clutches of that terrifying place.
Annabeth might find him. She's always been close to Percy and they often go on adventures together. So it's likely that she'd be the one to come across him when he faints.
Perhaps one of the Titans who were banished to Tartarus. They might see Percy as a threat to their plans of revenge or escape. Kronos, if he still has some form of existence in Tartarus, could be a major antagonist. He would have his own motives and powers, and he could try to manipulate Percy or simply get rid of him.
Annabeth might try to find a way to get to him. Their relationship is very strong, and she's not one to just leave Percy in danger. She could use her intelligence to figure out a way to enter Tartarus and help him escape.
One possible plot could be Percy having to face his inner demons in Tartarus. Maybe he has to battle new and terrifying monsters that are unique to that realm. He could also encounter old foes who have been made even more powerful in Tartarus.
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.
In fanfiction, if Percy is sent to Tartarus, his character development could be complex. He would likely become more self - reliant as he can't always count on his friends to save him. His view of good and evil might also change. He may realize that there are different shades of evil in Tartarus, not just the simple 'bad guys' he faced before. This could make him more empathetic in some ways, but also more cynical in others. He'd also grow in terms of his strategic thinking, as every move in Tartarus could mean life or death.