One consequence could be that his enemies might see this as an opportunity to attack him further while he's weak. But on the positive side, this could also lead to new alliances being formed in the fanfiction world. Other characters who previously had no connection with Percy might come forward to protect him, seeing his situation as unjust.
Another way is that Percy could have some sort of vision or gain new powers during his time of being wounded. Maybe the pain makes him tap into some latent ability that he didn't know he had, and this new development then drives the rest of the story as he tries to master it while also recovering.
Fan Xian had been seriously injured on his way back to the capital. There were many wounds and bloodstains on his body and he needed emergency treatment. His injuries were so serious that he needed the Eldest Princess 'carriage to stop outside the city gate and wait for treatment. In order for Fan Xian to receive treatment as soon as possible, Wang Qinian knelt down and asked the prince to make way. However, the First Prince was not willing to give way. He felt that this was a challenge to his authority. Fan Xian's injuries were serious and needed to be treated as soon as possible.
There's also the possibility that it's not a dark force at all. It could be a well - meaning but misguided wizard who accidentally causes Harry to be badly hurt. For instance, during a training session that goes wrong or while trying to perform a difficult spell to protect Harry but ends up harming him instead.
One common scenario could be during a battle with an akumatized villain. Maybe the villain has a new power that catches Adrien off - guard and he gets wounded while protecting Ladybug. Another situation might be in a non - akuma related accident. For example, he could be exploring an old building as Chat Noir and some part of the structure collapses on him. Also, in some fanfictions, there could be a plot where a new enemy specifically targets Adrien and manages to wound him during an ambush.
Often, a fanfiction might have Naruto severely wounded during a training accident that goes horribly wrong. He might be trying a new and extremely powerful jutsu that he's not fully mastered yet. This could result in him being in a critical condition. His friends and mentors then have to figure out how to heal him and deal with the consequences of his over - ambition in training.
One common problem is poor grammar and spelling. It makes the reading experience really difficult. Another is inconsistent characterization. For example, a character might act completely out of their established personality. Also, a badly written fanfiction may have a really weak plot. It could be just a series of unconnected events without a real purpose or direction.
One possible reason for Natsu to return badly hurt in a fanfiction could be a very tough battle. Maybe he faced an extremely powerful enemy that was far beyond his initial expectations. For example, if he encountered a new type of dark mage with some unknown and dangerous magic. This could lead to him getting seriously injured and having to struggle to return.
In the fanfiction, it might be that a mysterious figure is responsible. This person could have a vendetta against Amy's family or the stables she works at. They could set up a trap, like loosening a fence rail that Amy's horse jumps over, causing the horse to stumble and Amy to be thrown and badly injured.