Mildly Inconvenienced by the Narrative

So he keeps stealing Merlin away when Arthur needs him, makes comments about how he knows Merlin better than Arthur ever could and about how he knows Merlin sneaks away but he's loyal to Merlin over Camelot (which also upsets Arthur because that's one of his knights for god's sake!) and won't tell him where Merlin is going.

Arthur confronts Merlin about his sneaking away (magic things) and, when he refuses to tell the truth, Arthur thinks he can't trust him after all.

Merlin talks about having magic in front of S!Lancelot and, when he doesn't know what Merlin's talking about, Merlin becomes suspicious. He references something that didn't happen and S!Lancelot acts like he remembers that which confirms his suspicions but when he tries to sidle away out of the room to tell someone, S!Lancelot attacks him, attempting to choke him out.

Arthur finds them like this and S!Lancelot says that Merlin was sending information to Morgana so he had to subdue him which Arthur doesn't believe for ONE SINGLE MINUTE. Thus, they begin to duel as Merlin tries to recover his breathing and stop from passing out in the corner.

S!Lancelot attempts to kill Arthur and knocks him out, Arthur fading out with the last thing he sees being Merlin trying to push S!Lancelot's killing blow away as his servant stands.

Somehow (I'm thinking earlier in the ep Merlin finds a magical crystal that can hold souls released from the afterlife that Gaius is researching and Lancelot's real soul gets released during the ep and the crystal glows and maybe gets broken in the scuffle) Merlin pushes the Shade from Lancelot's body and his real soul is able to return to his body.

The Shade's form changes into a kind of Shadow Monster Lancelot and duels the real Lancelot now (who is very shaken and weak because he was just revived!) and, when he stabs the Shade, it does no damage. The Shade laughs and says that he can't be killed without magic and Lancelot says that then he'll just have to kill it like he killed the Griffin.

"You're a knight of Camelot and you're saying that you used magic to kill a Griffin?"

"No. I did." Merlin says, casting the same spell he did all those years ago on Lancelot's blade as he stabs through the Shade, killing it.

There's a beat where Merlin still isn't sure if it's the real Lancelot and asks him something only the real Lancelot would know, which he answers and they have a tearful/huggy reunion.

Arthur wakes up and they explain that it was a creature sent by Morgana and Lancelot is actually back with them now.

Later, when Merlin is getting Arthur ready for bed, Arthur tells him that he doesn't need to know where he goes off to because he trusts him. Merlin goes to tell him the truth but stops and thanks him instead, snuffing out the candles and shutting the door as he leaves.