Well, Arthur's reaction can be complex. At first, he might be angry because Merlin has kept this big secret from him. However, as he thinks about all the times Merlin has saved his life with magic, he begins to see that magic isn't all bad. He may feel betrayed, but also grateful in a way.
Arthur's reaction varies in different interpretations of the fanfic. He could be very confused, trying to reconcile the Merlin he thought he knew with this new Merlin who has magic. He might question his own beliefs about magic and his relationship with Merlin. He may also be a bit in awe of Merlin's power, especially if he has seen some really amazing feats of magic. As time passes, he may come to trust Merlin even more, seeing that Merlin's magic has always been in his and Camelot's best interests.
For Merlin, there are both good and bad consequences. On the bad side, some of the more traditional and anti - magic people in Camelot might try to oppose him. However, since Arthur now knows, he has a powerful ally. Arthur can protect Merlin from those who would do him harm. Also, Merlin can now be more open about using his magic for the good of Camelot, which is a positive change.
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.
Arthur might be shocked and immediately want to protect Harry. He could start by confronting those who are abusing Harry or find a safe place for Harry to stay.
Some fanfictions show Arthur becoming more dependent on Merlin's magic in certain situations. For instance, when facing a powerful enemy that can't be defeated by normal means. This changes the dynamic from just a king - servant relationship to more of a partnership. Merlin, on the other hand, might feel more confident in using his magic openly around Arthur, which also alters their interactions. They could have more equal conversations and decision - making processes.
The relationship could become more equal. Before, Merlin was in a somewhat subservient position, but with his magic out in the open, he becomes more of an equal to Arthur. They can now fight side - by - side in a different way, with Merlin using his magic to complement Arthur's skills. This new dynamic can add a whole new layer of depth to their relationship.
Often, Gwaine reacts with acceptance. He's not the type to shun Merlin just because of his magic. Instead, he'll likely embrace it as part of who Merlin is. He could say something like 'Merlin, your magic doesn't change a thing between us. You're still my friend.'
Arthur could also feel betrayed. He has trusted Merlin completely, and finding out about his magic might make him think that Merlin has been hiding something huge from him all this time. However, depending on how the fanfiction is written, he might eventually come to understand Merlin's reasons for secrecy.
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.
It would likely become very strained. Arthur has been brought up to uphold his father's anti - magic stance. So, when Uther discovers Merlin's magic, Arthur might initially feel betrayed and distance himself from Merlin.
Merlin might be initially shocked and heartbroken. He has been so close to Arthur, and this sudden forgetfulness would be a huge blow to him.