Well, Arthur could feel betrayed at first. He has been brought up to believe that magic is bad, and here his closest friend has been hiding this big secret. However, as he realizes how Merlin has saved his life and the kingdom many times with his magic, he might come to accept it and even see the potential of magic being used for the good of Camelot. For example, Merlin's magic has defeated many enemies secretly, which Arthur might now understand as a necessary protection for the kingdom.
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.
One possible reason could be a magical curse. In the world of Merlin, there are many dark sorcerers who might target Arthur. Another reason could be a poisoned weapon. If he was injured during a battle, the poison on the blade could make him sick. Also, it might be a result of some ancient plague that has resurfaced in Camelot.
One possibility is that Merlin's revelation of his magic could lead to a change in his relationships. For example, his friends who were previously unaware might react with shock, some with fear and some with acceptance. His enemies, on the other hand, would likely see him as an even greater threat and might plot more vigorously against him.
It could also be due to some past trauma. Perhaps Merlin has been through an experience where Arthur was involved in a way that now makes him flinch. Like, Arthur might have accidentally hurt Merlin during a battle training in the past, and now Merlin flinches whenever Arthur makes a sudden move.
One great aspect of Merlin - Arthur fanfiction is the exploration of their deep friendship. Writers often delve into the unspoken bond between them. For example, in some fics, they face new magical threats together that weren't in the original series. This allows for a fresh take on their characters as they rely on each other more than ever.
It could be that Gwen unknowingly has some sort of magical influence over Arthur. For example, if there is some ancient magic tied to her lineage that she's not aware of, and it's causing Arthur to focus only on her and forget about Merlin. Another aspect could be that Arthur's love for Gwen is so overpowering that it's subconsciously pushing out other important relationships, like the one with Merlin.
One possible reason could be a misunderstanding. Maybe Merlin did something that Arthur misinterpreted as betrayal or dishonesty. For example, if Merlin was secretly helping an enemy for a greater good that Arthur wasn't aware of at the time.
Merlin could choose a moment of deep trust between them. After they have shared some important experiences, like winning a difficult battle together. He might feel that Arthur is more open - minded at that time and would be more likely to understand his use of magic. Also, it could be that Merlin is tired of hiding such a big part of himself from his best friend and decides to take the risk of revealing it.
One great aspect of Merlin, Arthur and Guinevere fanfiction is the exploration of their complex relationships. Writers often delve into the unspoken feelings between them. For example, some fanfictions might focus on Merlin's role as a silent observer of the love between Arthur and Guinevere, while secretly having his own loyalties and emotions intertwined. It can also show different scenarios of how their story could have unfolded in a more magical or alternate universe setting, adding new layers to the well - known Arthurian legend.
I'm not sure about a very specific one, but you can check out Archive of Our Own (AO3). There are tons of fanfictions related to this. Just type in 'Journalist Arthur Magic Merlin' in the search bar, and you'll get a list. You might find some real gems there that explore different aspects of this unique concept, like how Arthur's journalistic skills help him in a world filled with Merlin's magic.