Merlin pretends to be possessed
I'm looking for a specific fic. I think I read it on fanficion.net, but it could be on AO3.
Merlin, Arthur, and the gang are kidnapped. Shenanigans happen and eventually, Merlin has to revel his magic. However, he pretends to be Emrys, (who he pretends is a separate person) who's possessing him, and that's why he can use magic, and that the two of them have an agreement for the sake of Arthur's safety.
They eventually get out, and Merlin pretends to wake back up. I think I remember that they use Kilgharrah to fly back to Camelot, and Arthur is surprisingly okay with the magic, I think.