User blog comment:Ḡwẵine Ḹٍٍkƨ Ĺiĸe Ͼềлȑềd/Arthur's Reaction After Learning Merlin's Magic/@comment-4650675-20120421210709

In the beginning of the show, I always thought that the show would end with Merlin saying to Arthur that he had magic. Then, Arthur would say something like "...I know."and then he would explain him that he realised while fighting him on 1x01. However, later I realised that Arthur was not as sharp as he looked at realising things, so he will probably learn the hard way.

In my opinion, he will react really bad. To him, magic has always been something bad. It killed his parents who he loved very much. Also, to him magic is the reason that his dear childhood friend and sister Morgana was corrupted and turned againist him. Now, if he learns that Merlin is the main reason that Uther died and Morgana turned againist him, he will probably never forgive this to Merlin,at least in the beginning. After he realises how much Merlin means to him, both as a force and as a friend, he will forgive him and accept him back, However, I think that it will happen in a way like, Morgana capturing Merlin and blackmailing Arthur that she will kill him, unless he hands Camelot(or his life) over, or something like that.