Green Lantern would use his power ring to create various constructs. He could create a large, impenetrable cage around Superman to stop him from escaping with Batman. He'd also use his ring to scan Superman for any signs of external influence, like magic or some sort of alien technology that's making him act this way.
It could start with a strange energy disturbance in Gotham or Metropolis. Ultraman appears out of nowhere and quickly subdues Superman and Batman with his unique powers. Then he just vanishes with them, leaving everyone in shock.
RWBY might be surprised by some of the darker themes in the fanfiction. Since their world also has its share of darkness, they could relate to the moral dilemmas faced by Batman and Superman.
One idea could be that Ultraman is from a different dimension where he's gone rogue. He sees Superman and Batman as threats to his new world - order. He uses his advanced alien technology to kidnap them. Maybe he wants to study their powers to make himself even more powerful in his own universe.
The Joker would probably be both delighted and a bit confused. He's used to Batman being his nemesis, but an evil Batman would change the dynamic. He might initially see it as a new and exciting game, but then realize that an evil Batman could be a real threat to his own chaotic existence. The Joker might try to manipulate the situation to his advantage, but it could easily backfire on him.
The Guardians would be in a state of panic. Bunny would be angry and ready to hop into action right away. He'd probably be the first one to want to charge in and rescue Jack. Tooth would be worried about Jack's well - being, not just physically but also emotionally. She might be concerned that Pitch could damage Jack's spirit. And Sandy, he'd communicate through his dreams. He could try to reach Jack in his sleep, giving him strength and hope.
Perhaps it's a case of a misunderstanding. Superman might think Batman has done something that endangers Metropolis, like developing a weapon that could potentially harm Kryptonians. He kidnaps Batman to find out the truth and keep the city safe. This could lead to a tense stand - off where Batman has to prove his innocence.
Robin would be extremely worried and confused. He would immediately start investigating what happened to Batman. He might suspect Poison Ivy right away and try to find her lair to break the hypnosis. Alfred, on the other hand, would be more cautious. He'd try to keep Batman's identity a secret while also trying to find a way to bring him back to normal, perhaps using some of his own scientific knowledge.
Wonder Woman would probably respect Batman's decision. However, she would also be concerned about the void his departure would leave in the team. She might start taking on more of a leadership role to fill the gap, while also hoping that Batman would return one day.
Sakura would probably be initially terrified. She would try to use her medical ninjutsu skills to defend herself or at least slow Itachi down. If she realizes she can't overpower him physically, she might try to talk to him, asking him why he's doing this.
She would be terrified at first, but quickly gather her wits. Hermione is very intelligent, so she would likely start observing her surroundings, looking for any possible means of escape or a way to send a signal to her friends.