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 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 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.
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.
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.
It could be a powerful sorcerer who wants to steal Percy's powers. Since Percy might have some unique abilities, this sorcerer sees her as a key to gaining more power for himself. He could be creating chaos in the city to draw Percy out and then try to defeat the Avengers when they come to her aid.
It could be an old enemy from their past, like a resurrected Titan or Giant. These powerful foes might manipulate Percy and Jason into betraying their friends as part of a grand plan to regain power or take revenge on the demigods.
It could be another powerful god or goddess who wants to use Percy's betrayal of Tartarus for their own gain. Maybe a god of chaos who sees this as an opportunity to create more disorder in the world. They might manipulate Percy into the betrayal and then turn on him later.
Loki could be a great main antagonist in this context. He's always been one to cause chaos and undermine the Avengers. He could manipulate events to make it seem like the Avengers are causing injustice, perhaps by framing them for crimes they didn't commit. His mischievous nature and magical abilities would make him a formidable foe.
A new character, like a powerful interdimensional sorcerer who wants to merge all the universes into one chaotic realm under his control. He has the ability to manipulate the fabric of the multiverse and can summon different versions of villains from various universes to do his bidding. The Avengers would have to not only fight his minions but also figure out a way to stop him from achieving his goal of multiversal domination.