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.
Another option is an ancient and malevolent god who was imprisoned in Tartarus long ago. Percy's return has somehow weakened the seals that held this god, and now it's slowly regaining its power. It sees Percy as the key to fully breaking free and is using all its minions and dark magic to either capture Percy or force him to do its bidding.
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.
He could come back haunted by the terrors he faced in Tartarus. In the fanfiction, Percy might struggle with nightmares and flashbacks. His relationship with Annabeth could be strained as he tries to deal with his trauma alone. But then, through the help of his friends and the love of Annabeth, he slowly starts to heal. Meanwhile, an old enemy from Tartarus could be following him back to the surface world, and Percy has to gather his courage again to fight it off and protect his loved ones.
Maybe a corrupted past version of a demigod. For example, a demigod who was promised great power by the Titans and turned against the Olympians. This demigod could see Percy as an obstacle to their plans and try to stop him at every turn. They would know the time period well and could use that to their advantage against Percy.
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.
Some of the more powerful and vengeful spirits in Tartarus could be the main antagonists. These spirits could possess demigods or mortals close to Percy to get to him. They might be angry that he escaped their domain and want to drag him back. Additionally, there could be a group of rogue demigods who believe that Percy has been corrupted by Tartarus and they see it as their duty to stop him, even if it means becoming his enemies.
Even some of the ancient demigods who turned bad could be antagonists. They might see Percy as a threat to their power or plans in the past. They could have different values and goals compared to Percy, leading to conflicts. For example, a demigod who was power - hungry and wanted to use time - travel to change things for his own gain, and Percy would have to stop him.
Maybe an old and powerful monster that has long been trapped in Tartarus could be the main antagonist. It sees Percy as a threat or a source of entertainment, and it tries to stop him from finding a way out or from getting stronger.
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.