Obviously, the Titan of Destruction would be the main antagonist. It could be a powerful and terrifying figure, with abilities that can cause mass destruction. It might have a horde of minions that Percy and his friends have to fight through.
Another possibility is that there is a group of ancient monsters that are awakened by the Titan of Destruction. They serve the Titan and are led by a particularly strong and intelligent monster. This group as a whole could be the main antagonist, with the Titan being more of a behind - the - scenes force.
Maybe it could be a corrupted sea nymph who has made a deal with the remnants of Titan power. She was once a friend to Percy but has now turned evil due to her greed for more power. She starts causing trouble in the sea, like making whirlpools that endanger ships and sea creatures. Percy would be heartbroken to have to fight her but also determined to stop her madness.
The plot might involve the Titan of Destruction targeting Camp Half - Blood. Percy and his friends have to come up with a plan to defend it. They could search for ancient artifacts that have the power to weaken the Titan. Along the way, Percy has to overcome his own self - doubts and fears.
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.