Hiding

Arthur develops magical abilities in his early teenage years and learns to hide it in fear of his father. Merlin comes along and they both try to hide their magic from the other while simultaneously trying to figure out what the other is hiding from them. Of course, they both need to hide their abilities from Uther, which becomes harder and harder after Merlin is kidnapped by an evil sorceress. All while they struggle to understand their feelings for one another.

Set in Season 1, sometime before The Beginning of the End, so he doesn't know he is Emrys yet, but Merlin has been Arthur's manservant for a while.

(I don't own Merlin or the characters, just loved the show)