Another possibility is that a new dark force, not related to Voldemort, emerges. This new entity could target Harry because they see him as a threat to their plans. They might have unique and powerful magic that they use to seriously wound Harry, leaving him in a state where his friends and the wizarding world have to figure out how to counter this new threat.
It could be a rogue Death Eater who has been hiding and waiting for the right moment to strike at Harry. They might use some forbidden dark magic to seriously harm him.
In some fanfictions, Harry could be seriously hurt during a dark magic attack by a new and powerful dark wizard. For example, he might be hit by a powerful curse that drains his magic and leaves him in a critical condition.
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's also 'When Pain Meets Comfort'. Here, Harry is suffering from the after - effects of a dark spell. James Potter, who has been observing Harry from a sort of spiritual plane, finally manages to break through and reach Harry. He takes Harry to a magical place where they can talk and James heals Harry's wounds, both visible and invisible, with his paternal love.
In some fanfictions, Harry's scar might hurt because of dark magic remnants. Maybe there are leftover spells from Voldemort that still linger and cause pain. It could also be a sign of a new threat emerging, like a hidden follower of Voldemort trying to use dark magic in a new way that affects Harry through his scar.
It could be a rogue Death Eater who managed to catch Harry off - guard. Maybe they were lying in wait and launched a surprise attack when Harry was alone, away from the safety of Hogwarts.
One example could be those where Harry is betrayed by his friends in Hogwarts. In some fanfictions, they might believe false rumors about him and turn their backs, which really hurts Harry as he values their friendship so much.
Another theme is the use of magic for healing. In many fanfictions, characters use spells or potions to heal physical wounds. But it also extends to emotional healing. For instance, a character might use a charm to soothe someone's troubled mind. Magic is not just a tool for fighting in these stories but also for mending what's broken, both in body and soul. And often, the process of using magic for healing becomes a moment of connection between the characters involved.
They're popular because they add a new layer of depth to the characters. We get to see how the beloved 'Harry Potter' characters handle pain and adversity, which makes them more relatable.
One popular 'Harry Potter Hurt' fanfiction could be 'The Aftermath of the Battle'. It focuses on how Harry copes with the physical and emotional wounds after the final battle. Another might be 'Injury and Healing' which delves into Hermione's experience when she gets seriously hurt during a dangerous mission and how her friends help her through it. There's also 'Harry's Hidden Pain' that explores the traumas Harry has endured throughout his years at Hogwarts that he never fully shows.
One plot could be that Harry is seriously injured during a battle with a new dark wizard who has emerged. His friends then have to find a rare magical herb to heal him while fending off attacks from the wizard's followers. Another could be that he gets injured in a mysterious accident during a dangerous potion - making experiment at Hogwarts, and it's up to Hermione to figure out the antidote before it's too late.