Perhaps a new evil force that was created from the very essence of Tartarus itself. A being that embodies all the darkness, fear, and chaos of Tartarus and wants to spread that to the world above. Percy would have to face this being that has no concept of mercy or pity.
The main antagonist could be Nyx. She is a powerful primordial goddess of the night, and Tartarus is her domain in a way. She might see Percy as an intruder and try to stop him from escaping or achieving his goals there.
Perhaps the antagonist could be a corrupted demigod. Someone who was also in Tartarus, but instead of coming out stronger in spirit like Percy, they were twisted by the darkness there. They now want to bring that same darkness to the world of the demigods and see Percy as the main obstacle to their plans.
Nyx could be a great main antagonist. She's a primordial goddess of the night and Tartarus is her domain in a way. She might see Percy and his friends as intruders and try to stop them from leaving or achieving their goals. She could use her power over darkness and the unknown to create all sorts of obstacles for them.
It could be a powerful demon that escaped Tartarus along with Percy. This demon might be bent on causing chaos and destruction in the mortal and demigod worlds. It has a personal vendetta against Percy for surviving Tartarus when it believes he should have perished there.
It could be another powerful god or goddess who wants to use Percy's betrayal of Tartarus for their own gain. Maybe a god of chaos who sees this as an opportunity to create more disorder in the world. They might manipulate Percy into the betrayal and then turn on him later.
Perhaps he could make an alliance with some of the less hostile creatures in Tartarus. There might be beings who are also tired of the chaos and want to help Percy in exchange for something, like a promise to free them from Tartarus if he ever gets the chance. Percy is known for his charisma and ability to make friends, even in the most unlikely situations.
In some more creative fanfictions, new characters might be introduced. For example, a powerful demigod who was born in Tartarus and has unique abilities. These new characters can add a fresh twist to the story, interacting with Percy and the others in unexpected ways, and often bringing new plotlines related to the mysteries of Tartarus.
Obviously, Percy Jackson himself is a main character. Annabeth might also be very involved, either helping him escape or being there to welcome him back. Grover could be another main character, perhaps using his nature - related powers to assist in some way.
There could also be some characters unique to the fanfiction. Maybe a new demigod who has some special power related to Tartarus and ends up joining Percy on his quest. Or perhaps an ancient being who was imprisoned in Tartarus long ago and has knowledge that Percy needs to complete his mission.
In some fanfictions, Percy Jackson might be depicted as dying in Tartarus. However, this is a departure from the original series. Fanfiction writers often take creative liberties, and some may choose to create such a tragic scenario for shock value or to explore different themes like sacrifice and the finality of death in a harsh, mythological setting.
Well, one aspect to consider in this fanfiction could be the character development. Percy Jackson is already a well - established character, but as the heir of Tartarus, he might face new challenges that force him to grow in different ways. Maybe he has to deal with darker powers within himself. Another point could be the world - building. Tartarus is a very dark and dangerous place in the Percy Jackson universe, so how the author expands on that and shows its influence on Percy and the story is important. Also, the relationships with other characters would be interesting. Will his friends still trust him when they find out he has this new 'heir' status? How will his enemies react?