Well, in this fanfiction, Percy controlling the Styx could lead to some really interesting plot developments. Since the Styx has such a significant role in Greek mythology, it could attract all kinds of mythical creatures and gods. Percy could find himself in the middle of a power struggle as different factions either try to use him or stop him. He might also use the Styx to travel to different realms or access hidden knowledge. And perhaps he would have to face the consequences of messing with such a powerful and sacred entity, like facing the wrath of Hades or other underworld deities.
In this fanfiction, Percy Jackson is the central figure as he controls the Styx. But other important characters could be Grover, his satyr friend. Grover has always been there for Percy and could provide support or comical relief. Also, Nico di Angelo could play a significant role. Given his connection to the underworld, he might have some insights or even a conflict with Percy regarding the control of the Styx.
One possible plot could be that Percy, using his powers over water, manages to control the waters of the Styx in a way that he can manipulate its boundaries. He might use this ability to keep out unwanted creatures or to create a safe passage for his friends. For example, when his comrades need to cross the Styx but are being threatened by the monsters that lurk in its waters, Percy steps in and uses his control to calm the Styx and guide them safely across.
He could emerge with enhanced physical abilities. The Styx is known for bestowing invulnerability in certain areas, and in Tartarus, this could be magnified. He might be able to withstand more powerful attacks from his enemies.
In this fanfiction, Percy might use his powers and leadership skills to bring order to the camp. Maybe he reorganizes the training schedules, making them more efficient for the demigods to improve their skills.
Annabeth could be a main character. She might be skeptical at first about Percy's new ability to control Styx, but then she uses her wisdom to help him understand the implications and possible uses of it. For example, she could research ancient texts to find out if there are any hidden dangers or benefits to Percy's control.
It could be an old enemy of Percy who has somehow gained power over the Styx. Maybe a resurrected villain from Percy's past who now wants to use the power of the Styx to take revenge on Percy and the gods. For example, Kronos' remnants could have found a way to manipulate the Styx's energy for their own evil purposes.
The Titans could still be a major threat. Even if Lady Styx is involved, the Titans might see an opportunity to use the chaos she causes to their advantage. For example, Kronos could try to ally with Lady Styx to regain his power and overthrow the Olympians once again.
In a fanfiction of Percy Jackson dipping in the Styx in Tartarus, it might explore the idea of him getting new powers or a stronger connection to his godly side. For example, his water - controlling abilities could become more refined and he could have a better understanding of the Underworld's waters. Also, he could encounter some long - forgotten spirits or creatures that guard the Styx in Tartarus.
Obviously Percy Jackson himself is a main character as it's about his dip in the Styx. His girlfriend Annabeth Chase would also likely be involved. She's always by his side in tough situations and would be there either to support him before the dip or to help him deal with any after - effects.
In the 'Percy Jackson Disappears' fanfiction, perhaps Percy could be kidnapped by an unknown enemy. Maybe it's a new villain who has a vendetta against the demigods. This would lead to his friends, like Annabeth, going on a desperate search for him, facing many dangerous challenges along the way.