Who could be the main antagonists in a 'Percy Jackson Mental Hospital' fanfic?

Perhaps a powerful monster in disguise as a doctor or staff member. They could be using the hospital's resources to create a portal to the underworld or something equally dangerous, and Percy has to stop them. They would be constantly trying to drive Percy crazy or keep him from interfering with their plans.

One possible plot could be that Percy wakes up in the mental hospital with no memory of how he got there. He starts to notice strange things like the nurses and doctors seem to have an otherworldly aura. As he tries to regain his memory, he discovers that the hospital is a front for some sort of mythological experiment. He then teams up with other demigods who are also trapped there to escape and expose the truth.

The Titans could still be antagonists. In the Percy Jackson universe, they are powerful and malevolent. A princess in a fanfic might have to face their wrath as they try to overthrow the gods and take over the world. They would see the princess as a threat, especially if she is allied with the gods or has some special power that could stop them.

A group of hunters who have a vendetta against wolves could also be antagonists. They might be descendants of some ancient hunters who were cursed by a god to always hunt wolves. Percy, as the king of wolves, has to protect his pack from them.

The Titans could be great antagonists. They might see Percy as a threat to their power if he becomes the God of Elements. Kronos, for example, could try to steal Percy's newfound elemental powers to regain his full strength and take over the world again.

The Titans could be antagonists. They might see Percy as a threat since he's Nyx's champion. They could try to stop him from fulfilling Nyx's tasks or even try to turn him against Nyx.

The Titans could still be antagonists. They might see Percy as a threat in his new role as the champion of Hecate. Another could be some rogue demigods who are jealous of Percy's new power and want it for themselves. And there could be magical creatures that have been corrupted by dark magic and oppose Percy's efforts to bring balance with Hecate's magic.

The traditional gods who don't want to see Percy as king could be antagonists. For example, Zeus might see Percy as a threat to his power and try to stop him at every turn.

There could also be some ancient monsters or spirits that are tied to Olympus' existence. They might rise up to defend it because their own survival is linked to the continued existence of Olympus. These could be powerful beings that Percy has never faced before, adding an extra layer of danger to his quest to destroy Olympus.

Obviously, the God of Death could be a main antagonist if he has some malicious plan. He might be trying to tip the balance of life and death in his favor, causing chaos in the mortal and demigod worlds.