I'm not sure specifically as I haven't read it. But generally, a Merlin fanfiction might be about the character Merlin from the Merlin series facing a great deal of pain or hardship in a fictional scenario created by the fanfic writer.
I don't know who the author of this particular 'a world of hurt merlin fanfiction' is. There are so many fanfiction writers out there, and without more specific information, it's hard to determine.
One common scenario could be Merlin getting hurt during a battle. Maybe he's protecting Arthur and takes a sword blow meant for him. Another might be Merlin getting injured while using his magic in secret and something going wrong, like a spell backfiring. He could also get hurt while on a quest in a dangerous place, like a dark forest where he trips over a root and injures his ankle while being chased by a wild beast.
One possible fanfiction could be that Merlin gets injured during a battle while trying to protect Arthur. Arthur then becomes extremely protective, refusing to let anyone else tend to Merlin's wounds. He stays by Merlin's side, worried sick, and vows to be more careful in the future to prevent Merlin from getting hurt again.
Well, in hurt Merlin fanfiction, a frequent theme is his isolation-induced hurt. Since he has to keep his magic a secret, he often feels alone. This can lead to him getting hurt in various ways, like being misunderstood. Some fanfictions also focus on the hurt he experiences when he fails to save someone in Camelot. It's a way to explore his character's vulnerability.
One of the themes is isolation. Merlin might get hurt and then be isolated either because he doesn't want others to know about his magic or because he fears being a burden. Also, self - sacrifice is a big theme. Merlin is often written as sacrificing himself for the greater good of Camelot, which leads to him getting hurt. This can range from taking a blow meant for someone else to using his magic in a dangerous way.
One good hurt Merlin fanfiction is 'The Broken Warlock'. It shows Merlin being physically and emotionally hurt after a major battle in Camelot. Another is 'Lonely Merlin' which focuses on his isolation and the resulting hurt. And 'Merlin's Heartache' is also great as it delves into his unrequited love and the pain that comes with it.
One popular 'knights hurt merlin fanfiction' might be 'The Wounded Mage'. In this story, Merlin gets injured during a battle while the knights are around. They then have to deal with the guilt of not protecting him well enough. Another could be 'Merlin's Hidden Pain' where the knights unknowingly cause Merlin emotional hurt through their actions.
This statement might be indicating that the amount of Merlin fanfiction that exists is all that there is in this 'world' of fan creations. It could be a comment on how the quantity has reached a sort of equilibrium. There may not be a great deal of growth in new Merlin fanfiction, or perhaps it's a realization that the existing body of work is all that fans can expect to have, without any significant expansion in the near future.
One place to find Merlin hurt fanfiction is Archive of Our Own (AO3). There are many talented writers who create such stories. You can search for tags like 'Merlin hurt' and explore the results. Some stories might involve Merlin getting physically injured during a battle or magically harmed, which adds a lot of drama and angst to the plot.
It's popular because it creates a lot of drama. The idea of the knights, who are supposed to be Merlin's friends, causing him harm adds an interesting layer of conflict.