Arthur means it as a joke. After Merlin wonders off again at the mall and it takes Arthur over an hour to track him down, playing with the plasma globes in the Discovery Store with the fascination of a child, Arthur makes a note to get him one for his birthday and then pulls him away from the store. "Honestly," he says, laughing, "some days I think I should just buy you a leash."
Merlin makes faces at him good-naturedly, and Arthur forgets about it until Merlin's birthday rolls around and Arthur decides to give him a gag gift, too. He makes Merlin open it first, then sits back, grinning, while Merlin rips off the gaudily-colored paper and lifts the lid off the box.
For a moment, Merlin is motionless, and Arthur is afraid. He's meant to laugh and joke, maybe roll his eyes and stick out his tongue in response to Arthur's teasing. Instead he just sits there, box in hand, staring at its contents.
Finally, Merlin sets the top down and draws out the gift. It's a simple black collar, with a D-ring on one end and a silver tag hanging off of it that says Merlin on one side and If found please return to: on the other, and Arthur's clipped a matching leash onto it to complete the joke.
Merlin runs his thumb over the silver grommets, then looks up and catches Arthur's eye. A slow grin spreads across his face. Arthur realizes all at once that he can breathe again. There's something more in Merlin's gaze than just amusement.
Arthur stares at him blankly when Merlin unbuckles the collar and holds it out to him.
"Well?" Merlin's eyes dance with merriment. "Let's see if it fits."
Arthur laughs, but Merlin's still holding it out, still waiting, like it wasn't a joke. Arthur's laughter fades to ringing silence. He takes the collar and holds it gingerly. It feels strange in his hands, cool and too heavy. When Merlin turns his back and drops his head forward, baring his neck, Arthur's mouth goes dry.
He lays his fingers against the back of Merlin's neck first, a light pressure. The fine tremor that goes through Merlin at his touch gives Arthur the courage he needs to slip the collar around his throat and buckle it. Not too tight, he cautions himself. Just secure enough that the supple leather molds itself against Merlin's skin.
Arthur's always had an appreciation for Merlin's throat, the long lines of it, the way it's broader and stronger than it seems at first glance. He's not prepared for the way the collar highlights every damn sexy thing about it until Merlin's got it on and it sends a punch of heat straight to his cock.
He reaches to clip the leash to the collar's D-ring, mostly because Merlin's eyeing it like he's waiting for it. As he does so, Merlin turns his head, rubbing his cheek against Arthur's hand, his eyes sliding shut in a blissful expression.
Arthur strokes his cheek, the corner of his eye. He traces a thumb around the edge of Merlin's ear and Merlin's lips twitch in the instant before he turns and licks at Arthur's fingers with quick, darting little laps. His eyes are hooded and his grin mischievous. Arthur's so distracted by the brief, blazing heat of Merlin's tongue on his skin that he's taken by surprise when Merlin bites his fingers, sharp enough to sting.
"Hey!" He snatches his hand back and frowns. "Stop that."
Merlin drops immediately onto his haunches, looking cowed. When Arthur reaches for him, he drops, rolling onto his back with his stomach bared. He wraps his arms around Arthur's and tugs at it, rubbing his face against Arthur's hand until he can't help but smile and pet Merlin's hair. When he does, Merlin makes an inarticulate noise that sounds like pleasure and gnaws carefully at his wrist.
Arthur sighs and smiles fondly down at him. "Silly pup," he murmurs, stroking his thumb over Merlin's brow.
Merlin's eyes light up. He flips over onto all fours and climbs into Arthur's lap, forearms braced against his chest and pushing him back in the couch. He nuzzles against Arthur's throat, laps and nips his way up to Arthur's ear, panting happily and wriggling just like a puppy. He's between Arthur's thighs, kneeling on the floor in front of the couch, and his writhing is doing serious damage to Arthur's ability to keep playing this game because jesus fuck, he wants nothing more than to bend Merlin over the coffee table and fuck him blind.
Merlin hooks his arms around Arthur's neck and hitches himself up to straddle Arthur's thigh. The angle is better now, he's closer, both of them on the same level. Arthur pushes his fingers into Merlin's hair and tries to pull him into a kiss, but Merlin turns his face aside at the last minute and bites at his jaw. He licks Arthur's mouth but it's playful, teasing, and when parts his lips and tries to coax him into a kiss, Merlin draws back, hands braced on Arthur's shoulders and elbows locked. His mouth hangs open, breath coming hard as he rocks against Arthur's leg — no, Arthur realizes, not rocking, humping. He drops his head back against the couch with a groan. His cock's so hard it fucking aches, but Merlin's relentless, making happy whining puppy noises as he gets himself off and it's so god damned hot Arthur can't bring himself to stop it.
He strokes his hands over Merlin's face, his hair, his jaw, the back of his neck, rubs his thumb over the bridge of Merlin's nose because it makes him push into Arthur's touch and pant harder, rut faster.
"Good boy," Arthur murmurs, stroking his throat, hooking his fingers through the collar as Merlin shudders against him. "That's my good boy."
Merlin whimpers and drives himself against Arthur's thigh. He twists, nudging at Arthur's arm until he lets go of the collar and Merlin can lick his hand. Arthur opens it for him, letting Merlin lick his palm until it's wet and sloppy and Merlin's eyes are heavy-lidded, his breath coming hard and fast.
"Merlin," Arthur says. It's an effort to make his tone sharp and commanding, but he does it, and is rewarded by the way Merlin responds to it. His eyes open and focus on Arthur with a fierce intensity, obedient and waiting. "Merlin, come."
Merlin's eyes fly wide and his mouth gasps open in the instant before his eyes roll back and he obeys, shuddering and jerking and still grinding himself against Arthur's thigh, coming with a strangled growl and the sharp bite of his teeth on the slope of Arthur's shoulder.
Arthur is half surprised he doesn't shoot off in his own pants, too, the way Merlin shudders and goes boneless against him and presses mewling little sounds against Arthur's skin as he comes down. He wants desperately, but Merlin curls against his chest with his head tucked under Arthur's chin and he can't bring himself to ruin the moment. He pets Merlin's hair as his breathing steadies and the sweat soaks through his tee.
When Merlin finally shifts in his arms and rouses, Arthur expects a laugh, a joke, expects him to take the collar off and make some crack about what just transpired. He expects him to abandon the game and speak. Instead, Merlin just nuzzles against his throat, nips him lightly, and then slithers down off Arthur's lap onto his knees. He noses at Arthur's crotch, pushes insistently against Arthur's throbbing cock with a sharp, distressed whine until Arthur lowers his fly and takes his cock out. Merlin swallows him to the root, sucks him eagerly, wiggling his hips like he's loving it too much to contain it.
Arthur slips his hand into Merlin's hair and bucks into his mouth. "Christ," he gasps. "Best birthday present ever."
Merlin hums his agreement and Arthur chokes, grabs fistfuls of his hair, and comes pouring down his throat.