Another possibility is that there is no single main antagonist. Instead, it could be a combination of misunderstandings, power struggles within the Hunt, and the influence of different factions. For example, some members of the Hunt might believe that Percy is a threat to their way of life or to their future plans. And these various factors combine to create the situation where Percy is betrayed, without there being one clear 'bad guy' driving the whole thing.
It could be a rogue member of the Hunt. Maybe someone who has a personal vendetta against Percy for some past event or just out of jealousy. This character might be secretly plotting against Percy within the Hunt and is the one who orchestrates the betrayal.
A powerful Titan who was once associated with the Hunt but turned against it. This Titan could be seeking revenge for being exiled from the Hunt in the past and wants to destroy it completely.
A possible main antagonist could be a rogue demigod who was once rejected by the Hunt and now wants revenge. This demigod could have allied with some powerful monsters and is constantly causing trouble for the Hunt. For example, they might be stealing important artifacts or sabotaging the Hunt's missions.
It could be Ares. He might see the Twilight vampires as a new source of power or chaos. He manipulates Percy into believing that the Twilight characters are evil and need to be stopped. So Ares becomes the main antagonist as he's the one driving Percy to betray the Twilight group.
It could be a jealous demigod. Maybe someone who has always been in Percy's shadow and decides to frame him for betraying the gods in order to get rid of him and gain more power and recognition among the demigod community.
A powerful and corrupted nymph could be the main antagonist. She was once a friend to the Hunt but was lured to the dark side by some ancient magic. Now she wants to destroy the Hunt and take over their territory. She uses her powers over nature to create all sorts of obstacles for the Hunt and Percy, if he's involved. She could also be very manipulative, turning some of the Hunt members against each other.
It could be a vengeful god. Maybe a god who feels that the sister's existence is an insult to their power. So they set out to stop Percy from finding her. For example, Hades might be the antagonist if he believes that the sister's return could disrupt the order in the Underworld.
A new, original villain could also be the main antagonist. This villain could be someone from Percy's past who has a grudge against him and wants to use the Avengers as a way to get revenge. By making Percy betray the Avengers, this villain can not only harm Percy but also cause chaos among the powerful superhero group.
It could be the gods. They often have their own agendas and might see the son of chaos as a threat to their power. So they might manipulate Percy into betraying the son of chaos.
It could be a jealous rival. This rival might have always been envious of Percy's popularity and skills. So, they manipulate Percy's friends into betraying him to get Percy out of the way and take over his position.
It could be an ancient evil god who manipulates Percy into betraying the son of chaos. This god might have a long - standing grudge against the son of chaos or his lineage and sees Percy as a useful pawn.