Merlin could be heartbroken. He might retreat into himself and become more withdrawn. After all, he has a deep bond with Arthur, and this sudden break - up would be a huge shock.
Merlin would be very worried. He would probably stay by Arthur's side day and night, using his magic secretly to try and heal him. He might also search for rare herbs or magical items that could cure Arthur.
Merlin is usually very empathetic. He'll listen to the reader's problems without judgment. In some fanfictions, he might share his own secrets and fears to make the reader feel less alone. He'll also do practical things, like bringing the reader food or making sure they have a warm place to stay if they're in a difficult situation. He'll try to show the reader that there are always options and that life is worth living.
He might also get angry. Merlin has his own pride and after all he has done for Arthur, being insulted like this would make him furious. He could start yelling back at Arthur, listing all the times he has saved Arthur's life or the kingdom.
Merlin react fanfiction is a type of fan - created story where the characters from Merlin are put in situations where they react to various things. It could be modern - day objects, events from an alternate timeline, or secrets being revealed. For example, Merlin might react to a smartphone in a modern - day AU (alternate universe) fanfic, with his magic causing some unexpected interactions with the technology.
The common people of Camelot would be delighted. They are used to seeing Merlin as the friendly servant, and now he is singing such a beautiful song. They would gather around him, their faces lit up with joy. For the magical creatures in the surrounding forests, they would be drawn to the magic in his voice. Some might come closer to Camelot than they usually do. Some might even try to communicate with Merlin, believing that he is a kindred spirit. This could cause some chaos as the people of Camelot might be afraid of these strange creatures approaching, but also lead to a new understanding between the two worlds.
Merlin would probably be very conflicted. On one hand, he is loyal to Arthur and Camelot, but on the other, being forced into a marriage is a huge infringement on his freedom. He might be scared about how this will change his life and his ability to use his magic secretly. Arthur, on the other hand, could be initially resistant as well. He sees Merlin as a friend and a servant, and this sudden change to a marital relationship would be strange for him.
Some might be shocked. For instance, Morgana, who has her own complex story arc, may be taken aback by how fans rewrite her motives in fanfiction. She might not expect certain sympathetic portrayals.
In some versions of this type of fanfiction, Arthur's reaction is complex. He might be stunned into silence when he first finds out. His mind would be racing with thoughts about all the times Merlin has used his magic to save him or Camelot without his knowledge. After the shock subsides, he may have a heart - to - heart with Merlin. They would talk about the implications of the secret, and Arthur would likely start to view Merlin in a whole new light, as not just his servant but as a powerful ally with a great burden.
Some of the knights may be filled with guilt. They might think that they should have been there to protect Merlin and prevent him from getting those scars. Others could be in awe, seeing Merlin in a whole new light, realizing that he has a hidden past full of pain and heroism that his scars represent.