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AU where Arthur finds out about the prophecy and pieces together that "his closest ally Emrys" must be Merlin because who else would it be, but incorrectly assumes that Merlin doesn't know and therefore has never practiced magic before.
Even better if he finds out early season 1 so He's just so devastated the entire time and he's just like "I can't believe this, this prophecy is going to turn my best friend into a sorcerer, and it's all my fault for being the once and future king" and Merlin's in the back just murdering a sidhe