Sight Unseen (слэш)

“Under no circumstances are you to follow me into that cave. Do you understand?” 
A pause. Merlin’s eyes are very blue, staring back at him, and Arthur resolutely does not notice the way they flicker down to his lips before returning to his face.
“I understand that you’re a giant prat, Your Highness,” Merlin mutters, finally looking away, but Arthur takes that for the agreement that it (sort of) is and smiles. And if his hand lingers for a moment at Merlin’s nape before he lets him go, well. There’s a not insignificant chance that he’s about to die alone and screaming; he might as well have one good memory to take with him into the dark. 
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OR: In which scary things happen in caves, as they are wont to do.