In the fanfiction, the environment of Tartarus itself could be considered an antagonist. The extreme heat, the toxic gases, and the ever - present danger of falling into bottomless pits. And on top of that, there could be some sort of demon - like creatures that are specifically created or summoned to hunt down Percy and Zoe and prevent them from achieving their goals.
The monsters native to Tartarus could also be antagonists. These could be huge, grotesque creatures that have never seen the outside world. They might attack Percy on sight, seeing him as prey. Some of them could be powerful enough to even give Percy a hard time, especially if his powers are weakened in Tartarus.
Well, one possible plot could be that Percy and Zoe have to face various ancient and terrifying monsters in Tartarus. They might encounter challenges that test their friendship and their individual strengths. For example, they could come across the Titans that were once banished there, and have to find a way to avoid or defeat them while also trying to find a way back to the upper world.
They might survive by relying on their combat skills. Percy, with his sword - fighting abilities and Zoe's archery skills, can fend off the various monsters in Tartarus. Also, they could use the environment to their advantage. For example, hiding in the dark corners or using the natural formations as shelters.
One possible plot twist could be that they discover a hidden power within Tartarus that allows them to communicate with the gods in a way they couldn't before. This new connection might lead to a deal where they can earn their freedom by completing a near - impossible task for the gods. Another twist could be that they find an ancient prophecy written on the walls of Tartarus that foretells their role in saving not just themselves, but the entire world from a greater threat that the gods are unaware of. Or perhaps they encounter a long - lost civilization of powerful beings in Tartarus who offer to help them escape in exchange for a future favor that will put Percy and Zoe in a difficult moral dilemma.
The Titans that were banished to Tartarus could be major antagonists. They might see Percy as an intruder and a threat, and try to stop him from escaping or even try to use him for their own purposes. Also, the various monstrous creatures native to Tartarus, like the Empousai or the Arai, could constantly be attacking Percy.
There could also be a group of demigods who are jealous of Percy's power due to his unique parentage. They might form an alliance against him, trying to steal his powers or at least make his life difficult. They could be led by a demigod who has a grudge against Percy or is power - hungry.
The Titans or Giants that were banished to Tartarus could be the main antagonists. They might see Percy as a threat or an opportunity to escape and seek revenge on the Olympians.
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.
One of the main antagonists could be the Titans that were banished to Tartarus. They are powerful and hold grudges against the demigods. For example, Kronos' influence might still be felt in Tartarus, and his minions could try to stop Percy and Piper.
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.