Perhaps one of the rival tribes or kingdoms sees T Challa as a threat and sends an assassin to hurt him. They might think that by injuring him, they can gain an advantage in the power struggle among different regions.
It could be a new villain who has emerged. This villain might have a personal vendetta against T Challa and specifically target him to cause harm.
It could be an evil magician. They might see Sadie as a threat to their plans, so they try to take her out by attacking her. For example, they could use dark magic to send shadowy creatures after her that would harm her.
It could be an alternate version of a character from his own universe. For example, an evil version of one of his friends or allies who has turned against him. In some fanfictions, a time - traveler from the future might come back and try to stop Barry from achieving something, and in the process, cause him harm. Also, a group of villains working together could be responsible. They could coordinate their attacks to overpower Barry and injure him.
The Riddler could be responsible. He might set up a series of elaborate and deadly traps throughout Gotham. Batman, in his attempt to solve the riddles and stop the traps from harming innocent people, could end up getting caught in one of them and getting hurt badly. For instance, a trap could involve a collapsing building or a room filled with sharp objects.
Sometimes, it could be due to her own actions. Lydia might be overconfident and take risks that she shouldn't. For instance, she could try to perform a very difficult and dangerous magic spell without proper training, and it backfires, causing her to get hurt.
It could be a villain in the fanfiction. Maybe a new, powerful vampire who has a vendetta against Bella or her friends.
Another character who might be responsible is a jealous rival. This person could be envious of Elena's popularity or her relationship with someone important. So, they set a trap that causes her to get hurt.
In the fanfiction, it might be someone from his own team. Maybe there's a new member who is insecure and feels that Kuroko is getting too much attention. So, in a fit of jealousy, this person does something that causes Kuroko to get hurt, like tampering with his equipment.
A new villain in the story could be responsible. This villain could be out to make a name for themselves by taking down Harley.
Perhaps an unknown ice sorcerer. Since the Frozen world has magic, a new character who is jealous of Elsa's power could target Anna to get to Elsa. They might use their ice powers to create dangerous situations for Anna, like making the ice floor slippery and causing her to fall and get hurt.
It could be Vilgax. He's one of Ben's most powerful enemies. Vilgax is always looking for ways to defeat Ben, and he might come up with a new and devious plan to hurt him. Maybe he teams up with other villains to ambush Ben when he's least expecting it.