User blog comment:Ḡwẵine Ḹٍٍkƨ Ĺiĸe Ͼềлȑềd/Arthur's Reaction After Learning Merlin's Magic/@comment-4135736-20120421183231

I think Arthur will learn not only of his magic but also that Merlin is Emrys, that he is a Dragonlord and that he poisoned Morgana which was one of the main factors of her downfall (I bet he will learn it from Morgana after she learns the truth about Merlin, it will happen while he and Merlin are out of Camelot), this is how I think Arthur will react, first he is in disbelief (angrilly accusing Morgana of lying), then he is in shock after Merlin confirms and proves that it is all true (seeing Merlin fight Morgana with his magic to protect him, forcing her to escape) and then he gets angry with Merlin (not talking to him at all when they return to Camelot).

In this theory, after he learns the truth about Merlin, Arthur summons Merlin to the courtroom and angrilly yells at Merlin for not telling him the truth, calling him a coward and blaming his secrecy for all the problems which have happened after Morgana turned evil. Merlin, in desperation, tells Arthur that he poisoned Morgana because she was the sourse of the sleeping enchantment, that he was unaware that she did not know who cast the enchantment on her, that Uther would have had him excecuted if he learn't the truth about him which was why he didn't reveal himself to Morgana, finally saying that he was unsure if Arthur was ready to know the truth about him.

Arthur, however is in an unforgiving mood as he is still resentful towards Merlin for his secrecy, he gets his sword (Excalibur) out but is unable to bring himself to kill Merlin or have him executed. Instead he chooses to banish him from Camelot and Merlin he does leave, going to his home villiage ealdor and falling into depression due to his guilt, believing that he could have at least attempted to tell the truth to Arthur, Morgana and everyone else before Morgana turned evil.

Arthur only forgives Merlin when Morgana, using Merlin's absence to her advantage, launches an attack on Camelot which makes Arthur tearfully realise how much Merlin meant to him and Camelot. He then goes to Ealdor and finds Merlin who snaps out of his depression and embraces Arthur, they apologise to each other and then return to Camelot to fight back against Morgana.