Merlin Pretends to be Possessed
I made a post a couple months ago searching for this fic, but I still haven't found it, so I thought I would give it another shot.
I believe I read the fic on fanfiction.net. It wasn't a very long fic, probably a one-shot or two-shot. I don't think it had any ships in it
Merlin, Arthur, and the Knights are captured, and put in a cell. I believe that it was Morgana who captured them, but it could just be a random villain.
Merlin is desprate to get them out, but can't think of a way to use magic without revealing he has magic. He decides to somewhat reveal his magic. He doesn't hide his glowing eyes when he casts magic, and when questioned, he tells them that Merlin and a powerful sorcerer named "Emrys" have an agreement, that when Arthur is in trouble, Merlin lets Emrys posses him/take over his body, to keep Arthur safe.
Arthur angrily questions "Emrys" demanding to know where Merlin was, to which Merlin replies that he's safe and just "sleeping" while "Emrys" is in his mind.
They escape, and Merlin calls Kilgharrah to fly them back to Camelot. He then pretends to "wake up." Arthur seems pretty chill with the whole magic thing. I think the knights were impressed with how dedicated to protecting the king.