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.
One common theme is recovery. After getting hurt, the story often focuses on how Jason Grace heals, both physically and emotionally. Another theme is the impact on his relationships, like with his friends in Camp Half - Blood or his romantic interests.
Jason could get hurt by a curse. His physical pain is accompanied by mental and emotional turmoil as the curse tries to corrupt his mind. His friends must find a way to break the curse, which might involve going on a quest to find an ancient artifact or a powerful magician. Along the way, they learn more about Jason's past and the origin of the curse.
Searching on Wattpad could also be fruitful. There are many amateur writers on there who might have created interesting Jason Grace hurt fanfiction. However, the quality can be a bit more variable compared to AO3. You might have to dig a bit deeper and use specific search terms related to the type of hurt (physical, emotional, etc.) and the relationships you're interested in within the story.
Piper might be very worried and use her charmspeak to try and keep Jason calm. Leo could use his mechanical skills to build something to help with Jason's injury, like a makeshift stretcher or a device to monitor his condition. Annabeth would probably come up with a strategic plan to get Jason to safety and figure out how to heal him.
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 great Jason Grace fanfiction is 'Jason's New Quest'. It takes Jason on an adventure that's not in the original books, with new characters and challenges. It really delves into his character and shows his growth in different situations.
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.