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.
There are many possibilities. For example, in the 'Harry is badly hurt fanfiction', it could be that there is a malfunction in a magical experiment he's involved in. The magic backfires and causes him great harm. Or, there could be a plot where a new, powerful and evil force has emerged in the wizarding world and targets Harry specifically. During their encounter, Harry is badly hurt while trying to fight back or escape.
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.
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.
One could be 'Jason's Painful Ordeal' where he gets seriously injured in a battle against a powerful new foe. His friends are left to deal with the aftermath, and it shows their grief and determination to save him.
To write a great fanfic about Jason being badly hurt, you need to research. Look at how other fanfics handle injuries. For the story, start with a strong opening that shows the moment of injury vividly. Then, develop the plot around his recovery. Include his friends' reactions, like Annabeth's strategic thinking on how to help or Percy's concern. And don't forget to add some internal monologue for Jason to show his pain and determination.
There could be a fanfiction where Jake Gyllenhaal is an athlete in the story and he gets hurt badly during a competition. The story might detail his physical pain, the disappointment of not being able to finish the event, and his journey to regain his strength. It could also involve his interactions with his fans who are worried about him and how he appreciates their support during his recovery.
One common theme is the idea of overcoming adversity. When he's hurt badly in the fanfiction, the story often focuses on how he battles through the pain and recovers. Another theme is the support he gets from others, like friends or family. It shows how important relationships are in difficult times.