It started out as an accident. Arthur had asked Merlin, quite innocently, to speak some dragon tongue. He'd just wanted to understand it a bit, that was all.
But then he'd heard the words rolling off Merlin's tongue, felt the weight of them wash over him, the deepness and richness that Merlin's voice had taken on. This was not the voice of one who asks. This was the voice of one who commands.
And he was instantly, painfully hard.
He'd tried to hide it, but Merlin was sharp-eyed and noticed more than most people gave him credit for, especially where Arthur was involved. The moment he saw the glitter in Merlin's eyes, Arthur knew he was in trouble.
Now Merlin did things like bend over at the dinner table, his mouth a tad too close to Arthur's ear, and murmur "Géate cyre," making Arthur's spoon clatter to the tabletop.
Now he panted car grise áþes into Arthur's mouth as they kissed, causing Arthur's knees to buckle.
Now he gasped mé tácen átende diegollice into Arthur's earlobe, his shoulder, his neck as Arthur thrust into him, making Arthur stutter and spill violently.
Sometimes Arthur asked for a translation, and sometimes he didn't. He suspected that a lot of the time Merlin just said the first thing that came into his head, even if it had nothing to do with the situation. Because it wasn't the meaning of the words that drove Arthur crazy. It was the sound of them, and the way Merlin got when he spoke them.
It was, in fact, Merlin himself.
(And besides, it was much less embarrassing than some of the other kinks he heard about.)
(Shut up.)