In the fanfic, it might be that Percy was in the wrong place at the wrong time. He could have stumbled upon something he wasn't supposed to see, and the abusers wanted to silence him. Or it could be that there's a prophecy or some sort of future prediction related to Percy that scares others, so they abuse him in an attempt to prevent that future from happening. Also, there could be some internal conflict within a group he belongs to, and some members use abuse as a way to push Percy out or subdue him.
Perhaps there's a power struggle in the story world. Percy might be seen as a threat to certain groups or individuals who then resort to abuse as a means to gain control over him. Another reason could be related to his family background. If his family has some sort of enemies, they might take out their anger on Percy.
It could be one of his close friends. In many fanfics, friends often play a crucial role in uncovering such secrets. They might notice Percy's unusual behavior, like being overly jumpy or withdrawn, and start to suspect something was wrong.
One reason could be to explore complex psychological and emotional states. By making Percy abused and mute, the writer can delve into how a character copes with trauma and isolation. However, this is a very dark approach.
They might be shocked at first. They could feel a sense of guilt for not noticing earlier. Maybe they immediately try to comfort Percy and offer their support, like giving him a safe place to stay and assuring him that he's not alone anymore.
The others in the seven, like Jason, would probably feel a sense of guilt for not noticing earlier. They would come together as a group to support Percy. Jason might offer his friendship and strength to Percy, telling him he doesn't have to face it alone anymore. They would also likely start to investigate who was responsible for the abuse.
It could lead to a stronger bond among the seven. They would become more united in protecting each other.
One possible plot development is that the gods gather to hold a trial for the abuser. They would bring in witnesses, and Percy would have to testify about what happened to him. This would be a very emotional and intense scene.
It could have a negative impact. It might make some fans uncomfortable or even trigger those who have had similar experiences. It could also change the way people view the characters in a way that may not be in line with the original intended portrayal.
Some authors might write it to add a new and unexpected twist to the Percy Jackson story. They may want to see how the other characters would react if such a thing happened. Also, it could be an attempt to explore the psychological impact on Percy in a very different and extreme situation, though this is a rather controversial approach.
The abuser could face immediate death. Gods like Hades might be quick to drag the abuser down to the Underworld for punishment.
Well, Percy could experience emotional trauma. This might show up as nightmares or flashbacks in the fanfiction. He might also isolate himself from his friends and family because he feels ashamed or doesn't want to burden them with what he's been through. And his view of the world could become more cynical. He may start to see the world as a cruel place where bad things happen to good people like him.