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Wait, wait, wait, Merlin AU where instead of coming up with disguises as Dragoon and Dolma, Merlin and Gauis just pretend there's this magical spirit called Emrys who helps out. Because like Emrys is basically magic incarnate, right? so it could be easy to bs the idea that magic incarnate has no physical form.
Anyway, so now whenever magic help is needed they're like ah get Emrys but thing is Emrys needs a host to help out on the mortal plane or some bs like that and he always possesses Merlin. Thus allowing Merlin to help out without Arthur realising Merlin is helping out.
Plus then if they ever run into the druids and they're like "yo Emrys we love you" to Merlin, he can just be like "oh yeah he possessed me and dragged me out here the other day, they think I'm him oops".
Also if they're ever in a pinch then Merlin can just use magic and be like "ah looks like Emrys was around then".
Plus instead of the tavern excuse, it upgrades to the Emrys possessed him excuse whenever Merlin's off doing something.
Eventually it reaches a point where 'Emrys' has just saved them all and Arthur has completely had it with this magic spirit that keeps stalking him, possessing his manservant and dragging his manservant off into dangerous situations so instead of thanking him he just stands there yelling for ten minutes and Merlin is just trying to keep a straight face the entire time.