It could be an old enemy of the gods who has been biding their time. Maybe a powerful Titan that was thought to be defeated but has found a way to come back. This Titan could use Percy to kidnap Reyna because she has a connection to a certain power that the Titan wants to use to regain full strength and overthrow the gods once again.
Perhaps it's an ancient, malevolent spirit that has been awakened. This spirit has the ability to possess Percy and make him do things against his will, like kidnapping Reyna. The spirit's goal could be to use Reyna as a sacrifice to open a portal to a realm of darkness, from which it can summon an army of dark creatures to take over the world of the demigods.
One possible plot could be that Percy is being controlled by some dark force. Maybe a new evil god or an ancient curse. This force makes him kidnap Reyna because she has some important knowledge or power that the dark side wants. For example, she might know the location of a powerful weapon that could tip the balance in a coming war between the gods and the titans' remnants. Percy, against his will, takes Reyna to a secret location. Reyna, being a strong and resourceful character, tries to figure out a way to break Percy out of the control while also dealing with the situation at hand.
It could be an old enemy of the gods who has a vendetta against both Camp Half - Blood and Camp Jupiter. This antagonist might see Percy and Reyna as threats because of their combined power and influence among the demigods. For example, a powerful Titan who escaped from Tartarus and wants to use the chaos between the two camps to his advantage.
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.
Perhaps an ancient and dark deity who has been awakened due to some event related to Percy's exile. This deity wants to bring chaos to the world of gods and demigods, and Percy, even in exile, is the only one who can stop it. The deity would use all kinds of dark magic and minions to oppose Percy and his friends.