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.
The first thing that could happen is that his friends would immediately start to tend to his wounds. Erza, being the strong and reliable one, might take charge of the situation. She could use her medical knowledge, if she has any in the fanfiction world, or she could call for help from a more experienced healer. This would show the bond between Natsu and his friends.
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.
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 could be a misunderstanding during a mission. Maybe Erza's actions were misinterpreted by Natsu, leading to him getting hurt, either physically or emotionally.
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 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.