Nothing.

[Oh look, he can still lie to a Pendragon and make it look easy. But the sound of her voice reminds Merlin that she's not been well. First Sam and now Arthur...he suddenly hates this city and its wild magic that takes people from them.]

There's just someone I need to find.

[Not 'look for'. Find. It can't be any other way.]

I'm sorry that I have to ask you for a favor, but there's no one else that I can trust.

[ Someone who needs to be found is someone who is lost, Merlin. What - ]

Who? I shall do as you ask of me; but give me a name. Is it the Mist again? Merlin you cannot go by yourself.

[He doesn't really mean to go silent for as long as he does. But he's found Arthur's letters, and he knows he should take hers to her before he leaves. It's all enough to make Merlin's knee's go suddenly weak. But he won't cry. He's not about to prove Arthur right this late in the game. But he does sound resolved. Tired.]

It's Arthur. He wasn't in his bed this morning.

[And he knows it's foolish, reckless to go looking for him. Arthur didn't just wander off into the Mist without his sword or armour. But Merlin has to do something, and this is all he knows.]

Take care of yourself while I'm gone. If you see Gwen, tell her not to follow me.

[ A moment of silence. And another. And a third as she lets it sink and no; not Sam and my brother both at once, no. ]

You cannot go alone. Merlin. I cannot lose you, too. You cannot.

Maybe I'll...ask someone to go with me. At least part of the way.

[Though it takes him a moment to come up with a name or a face. Rukia, Lancelot, Arthur. They're all-]

Aithusa and I are leaving now. We should be at your place in no time.

Keep safe. Merlin, promise me. Promise me you'll come back.

...I promise.

[It's probably wrong of him to make a promise like that. But he'll do everything in his power to keep it.]