A group of dark forces or minor gods could be the antagonists. They might believe that the union of Percy and Artemis would disrupt the balance in a way that benefits them. So, they engineer the situation where Percy is forced to marry Artemis, hoping to use the resulting chaos to their advantage. They would likely try to keep Percy and Artemis from finding a way out of the forced marriage.
The Olympian council itself could be the antagonist if they are the ones forcing the marriage for their own political gain. They might not care about Artemis' wishes or Percy's attempts to stop it, and they use their power and influence to push the marriage forward.
Well, one possibility could be that some powerful entity forces Percy into this marriage for political reasons. Maybe there's a power struggle in the mythological world and this union is seen as a way to bring stability. For example, Artemis might be against it initially because of her vow of maidenhood, but then she and Percy find a way to work together against the forced situation.
A great main antagonist could be a powerful Titan who has somehow escaped from Tartarus and is seeking revenge on the gods. He could target the Hunters of Artemis because they are loyal to Artemis, a goddess he has a grudge against, and Percy because of his connection to Poseidon.
A possible plot twist might be that the force behind the marriage is actually an ancient evil that wants to use their union to gain power. Artemis and Percy then have to team up to not only deal with the unwanted marriage but also defeat this evil. It could also be that Artemis has a way to break the forced marriage bond, but in doing so, she would lose her immortality, which adds a huge moral dilemma to the story.
It could be a jealous demigod. Maybe someone who has always been envious of Percy and Annabeth's relationship and popularity. This person finds a way to manipulate some dark magic or an old rule to force them into marriage, hoping to create chaos in their lives.
Artemis would be in shock initially. Given her status as a powerful and independent goddess, the idea of being forced into marriage is unthinkable. She would likely try to use her divine powers to break free from the situation. If she can't do it immediately, she might start to negotiate. She could offer other things in exchange for her freedom from this marriage. And if Percy shows any sign of taking advantage of the situation, she would not hesitate to put him in his place.
One possible plot twist could be that the gods interfere to stop the marriage at the last minute. They might realize that such a union goes against the natural order. Another could be that Percy discovers a way to break the forced bond by finding a magical artifact. Or perhaps Artemis herself finds a loophole in the situation that allows her to avoid the marriage without causing chaos among the gods.
Maybe the main antagonist is an ancient evil that has awakened. It has the power to manipulate Artemis's mind, making her kidnap Percy against her will. This evil entity wants to use Percy as a sacrifice to gain more power. So, while Artemis is the one who physically kidnaps Percy, the real force behind it, the ancient evil, is the true antagonist.
Perhaps a powerful and corrupted demigod could be the main antagonist. This demigod might be jealous of Artemis' power and popularity among the other demigods. He devises a plan to harm her, and Percy, being loyal to Artemis, has to give his life to save her from this rogue demigod.
It could be a powerful Titan who has escaped from Tartarus and blames Artemis for something in the past. So he's out to get her, and Percy has to stop him at all costs, even if it means his death.