Maybe it's a dark entity that feeds on chaos. Since Sarah's power of control is a threat to the chaos it thrives on, it becomes the main antagonist. It would try to corrupt Sarah, make her lose control so that it can continue to spread disorder throughout the fantasy world.
It could be an evil sorcerer. This sorcerer might be jealous of Sarah's unique power of control and wants to take it for himself. He could constantly send his minions to attack Sarah and try to disrupt her journey of self - discovery and power - mastering.
There could be a powerful demon from the underworld. This demon has heard about Sarah's power of control and believes that if he can take it from her, he can rule not only the fantasy world but also break through to the human world. So he sends his minions to harass Sarah and tries to find her weakness so that he can defeat her and steal her power.
In a 'Controlling Sarah' fantasy story, perhaps Sarah discovers she has the power to control the elements. She could start by taming wild fires that threaten her village. With her newfound power, she has to learn to control it so as not to cause chaos. Maybe she gets mentored by an old wizard who teaches her the ethics of power control.
Maybe there is a dark force within the Goblin Kingdom itself. Some of the goblins might not be happy with Jareth having a half - human baby as his heir. They could plot against the baby and Sarah, trying to undermine Jareth's rule and get rid of the baby. Sarah and Jareth would then have to deal with this internal threat while also protecting their baby from outside forces.
It could be a disgruntled client. Maybe the female attorney didn't win a case for this client as they hoped, and in revenge, they dig up some dirt on her to blackmail her.
Perhaps it could be an ancient god who has been awakened from a long slumber and wants to take over the world. This god would have powers that rival Wonder Woman's.
The ex - husband is most likely to be the antagonist in such a blackmail ex - wife fantasy story. He might be bitter about the divorce and think that blackmailing his ex - wife is a way to get what he wants, whether it's money, property, or revenge.
It could be someone from the Naruto universe who doesn't like Naruto's newfound fascination with Aku. For example, Danzo might see Naruto's interest in Aku as a threat to the stability of the village and try to stop Naruto from further exploring Aku - related things. So, Danzo could be the main antagonist in this case.
The plot could involve Sarah discovering she has some special power in a magical world. Maybe she can control elements like fire or water. And she has to use these powers to save her land from an evil sorcerer.
A powerful dark sorcerer could be the main antagonist. He could be jealous of the heroes' quirks and see them as a threat to his magical dominance. He might start by cursing the heroes or stealing their quirks to add to his own dark magic.
Perhaps a group of anti - technology extremists could be the main antagonists in an Ultron fanfic. They see Ultron as the symbol of all that is wrong with technology and are determined to take him down, no matter the cost. They might use their own advanced, but more rudimentary, technology to fight against Ultron. This would create an interesting contrast between Ultron's high - tech abilities and their more basic, yet effective, methods.