Who could be the main antagonist in a 'Percy Jackson King under the Mountain' fanfiction?

A group of rogue gods who oppose the idea of the Phoenix King existing. They feel that his power is unregulated and could disrupt the balance of power among the gods. They plot to eliminate the Phoenix King and anyone who stands in their way, which includes Percy and his friends. They use their godly powers in devious ways to create problems for the Phoenix King and his allies.

It could be a powerful sorcerer who wants to control the Hellhound King for his own evil purposes. This sorcerer might be trying to use the Hellhound King's strength to take over Camp Half - Blood or other important places in the Percy Jackson world.

It could be an ancient and powerful dragon that has been awakened by the arrival of Percy in Alagaesia. The dragon might see Percy as a threat to its rule or territory and decides to go after him. This dragon could also have some special abilities that make it difficult for Percy to defeat, like being able to nullify his water powers for a short time.

A powerful Titan who wants to use the Hellhounds for his own evil purposes could be the main antagonist. He might try to capture Percy and force him to command the Hellhounds to do his bidding, like attacking the demigod camps.

Well, perhaps in the fanfiction, Percy is summoned by the 'King under the Mountain' because of his reputation. The king wants Percy to retrieve a powerful and ancient artifact that was lost in a dangerous part of the mountain. Percy teams up with some of the king's bravest warriors. Along the way, they face all sorts of mythical creatures like trolls and goblins. Percy has to use his wits as well as his powers to overcome the obstacles and claim the artifact.

The obvious choice for the main antagonist could be a powerful and ancient monster who has the ambition to be the king of all monsters. It could be a Titan - like creature that has been in hiding for centuries, gathering strength. Maybe it's a monster that was once defeated by the gods but has now returned stronger than ever.

It could be a power - hungry Atlantean noble who wants to rule not just Atlantis but also the surface world. He sees Percy as a threat to his plans and tries to stop him at every turn.

A new and powerful demon that has emerged from the darkest corners of the underworld. This demon is bent on using Percy's connection to the overworld to break through the barriers between the two worlds and unleash chaos. It has powers that are different from what Percy has faced before, like the ability to manipulate the very essence of the underworld's environment.

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 it could be a demigod who has become obsessed with power through gaming. This demigod, let's say a child of Hades, discovers a way to use dark magic and combine it with gaming technology to try and rule over all the other demigods. He sees Percy as a major threat to his plans because Percy is so popular and has strong powers, so he constantly tries to undermine Percy at every turn.