Who could be the main antagonist in a 'Percy Jackson Closes the Doors of Death' fanfiction?

Maybe a group of rogue demigods who have been corrupted by dark magic. They believe that by keeping the Doors of Death open, they can gain immortality or unlimited power. Percy would then have to face his own kind in order to close the Doors.

Well, it could involve Percy facing numerous challenges in the underworld. Maybe he has to battle new and powerful monsters that guard the Doors of Death. He might also get some unexpected help from his friends, like Annabeth using her wisdom to figure out the secrets of closing the doors. And there could be a final showdown with some major villain who doesn't want the doors to be closed.

It could be a new, powerful and malevolent god. This god might be jealous of Percy's status as a hero and the favor he has from some of the Olympian gods. So, the new god devises a plan that makes Percy's life so miserable that he asks for death.

One idea could be Percy having to face his inner demons while closing the Doors of Death. Maybe past failures or losses resurface, adding an emotional depth to the task.

A powerful demigod who has been corrupted by the idea of primordial power could be the main antagonist. They might believe that by allying with a primordial or obtaining their power, they can become greater than the gods. This demigod would then oppose Percy, who stands for the protection of the current order.

A powerful Amazonian sorceress who has turned against her own kind and is now trying to use dark magic to take over the world. She could see Percy as an obstacle in her path and constantly try to defeat him.

Maybe a rogue demigod who blames Percy for something and wants to make his exile permanent. This demigod could have some dark powers or be in league with some evil forces. They would constantly try to thwart Percy's attempts to return or find a way to clear his name.

Maybe an ancient sea demon that has possessed a pirate captain. This possessed captain is now leading his crew to cause chaos everywhere. They are attacking other pirate ships and even some demigod - related places. Percy has to find a way to exorcise the demon from the captain and stop the crew's rampage. The demon is very cunning and uses the pirate's knowledge of the sea to its advantage.

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.

It could also be a powerful sorceress who has her own motives for disrupting the demigod world. She might be seeking to gain control over the Olympian gods by using Percy as a pawn. She could use her magic to manipulate events and people around Percy, causing chaos and confusion.