A demigod who was jealous of Percy's connection to the timeless warrior concept. This demigod makes a deal with a dark force and becomes a twisted version of a timeless warrior, with powers that are a perversion of what they should be, and is determined to take Percy down.
A powerful Titan who was once a timeless warrior but was banished to the depths of Tartarus could be a great antagonist. He has spent centuries plotting his revenge and now wants to use Percy to break free and destroy the Olympians once and for all.
A powerful chaos deity could be the main antagonist. This deity might see Percy as a threat to their plans of spreading chaos throughout the world. They could constantly send their minions to attack Percy and try to corrupt his mind.
It might be a new, self - made god. This being has risen from the ashes of human greed and ambition. He sees Percy, the immortal warrior, as a threat to his plans of world domination. He uses his own brand of dark magic and his army of mind - controlled minions to try and defeat Percy. He also tries to turn the other gods against Percy by spreading lies and false prophecies about him.
Perhaps an ancient and malevolent sorcerer who has been awakened from a long slumber. The sorcerer wants to use Percy's powers for his own dark purposes. He sets traps and sends magical creatures to stop Percy as he tries to figure out what the sorcerer is up to and how to defeat him. The sorcerer would be a very tricky antagonist as he can use all kinds of magic that Percy has never faced before.
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.
The goddess Hera might also be a great antagonist. Since she has a history of interfering in the lives of demigods, she could be jealous or threatened by Percy's rebirth. She might try to stop him from reaching his full potential in his new life or cause trouble between him and his new allies.
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.
An ancient and forgotten god who has been awakened. This god could be angry at the modern demigods for ignoring the old ways and decides to send assassins after Percy as a warning to the rest. They might want to bring back the old order where gods were feared and worshiped more and see Percy as a symbol of the new, more independent demigods.