User blog comment:Ḡwẵine Ḹٍٍkƨ Ĺiĸe Ͼềлȑềd/Arthur's Reaction After Learning Merlin's Magic/@comment-4640114-20120423020715

I think Arthur might be attacked by Morgana or Mordred and Merlin will be forced to use magic to defend Arthur. So Arthur will find out very suddenly as Merlin is dueling against the enemy that he has magic. Arthur will probably just sit there like an idiot for a minute than he'll say somethin stupid like, "You have magic?!!!" And Merlin won't even look at him. Then Arthur might get kind of mad and be all like, "All of this time and you've been lying to me!" Merlin will look over at Arthur then Morgana or Mordred will take that oppertunity to try to attack Arthur again. Merlin of course will jump in front of Arthur taking the blow and will get seriously injured. The knights will show up and Morgana or Mordred will be forced to flee. The knights will go after him/her and Arthur will rush to Merlin. He'll take him back to the castle and Merlin will be treated. Merlin will heal but it will take a while, giving Arthur time to think about magic as he feels guilty for Merlin getting hurt and soon he will make it legal.

Just a theory of course. :)