Arthur just couldn't help himself, after Merlin's admission that he'd never done anything like this before, that he'd never let anyone but Arthur use him like this. A few illicit blowjobs here and there, a couple of furtive, fumbling hand jobs with boys back in Ealdor, and that was the sum total of Merlin's experience; for some reason, that had been the biggest turn on for Arthur.
For instance, now, with Merlin in his bed, face down in the pillows, up on his knees with his arse in the air, Arthur just wants to fuck him harder, grip his hips more tightly until his hands bruise the pale skin there, marking Merlin forever. He wants to imprint himself on Merlin, make sure Merlin never looks elsewhere for this, wants to fuck Merlin until he knows exactly where he belongs – under Arthur in his bed.
So that's exactly what he does.
He loves doing this to Merlin, fucking him until Merlin can't see straight, can't sit down the next morning, loves watching the catch in Merlin's stride and knowing that he put it there, that he marked Merlin in a way no one else will ever really see. Oh, they'll see the way Merlin holds himself carefully, can't walk properly, and they'll know that the Prince has had him again, and they'll gossip about it, but they'll never see the way Merlin's raw-fucked hole puffs around his cock, or the bruises on his hips, the bite-marks Arthur gives Merlin on his shoulder blades. Arthur does like marking Merlin visibly – just loves watching those pale cheeks flush red as people ask him about the bite marks on his neck – but for some reason, it's the marks no one will ever see which please him more. Merlin is his in ways no one else will ever know, and in ways no one else will ever – has ever - experienced.
That thought takes him over the edge, and he comes inside Merlin with a grunt, for the second time that night, reaching round under his manservant to jerk him off with rough, unstudied movements, relentless until Merlin comes himself, a few moments later, with a cry that is pulled almost unwillingly out of his throat. His knees give way, and Arthur lands on top of him as he sprawls onto the bed, the prince's weight forcing him deeper into the mattress.
For a moment, Arthur takes his time to enjoy having Merlin under him, toys with the idea of keeping him that way until he's ready to take him again – it's not like Merlin doesn't enjoy that sort of thing, now that Arthur has taught him about it – but in the end, he rolls both of them over and manoeuvres Merlin (sated and pliant) until he's sitting in Arthur's lap with Arthur's cock still inside him, compliant to a fault.
Arthur takes a moment to be tender with his lover in a way they aren't during sex, stroking one hand up his thigh and asking, voice low, "Are you alright?"
Merlin's smile is warm and satisfied, "Oh, yes." He says fervently, "Better than alright."
"Good," the soft stroking becomes a stinging smack, and Merlin jerks, not wholly surprised but not quite expecting it either, "You can ride me again in a minute."
It doesn't take long for Arthur's cock to harden again – but then, with Merlin in his lap looking like that, the muscles of his arse clenching around his cock with every shift and movement, it would have been more surprising if it had taken long. Arthur lands another smack on Merlin's thigh, admiring the way his handprint stood out red on the pale skin, and nods at him. "Ride me," he orders, and Merlin obeys, bringing his knees under him on the mattress and forcing himself up on Arthur's cock.
Watching him is gorgeous, watching him fuck himself on Arthur's cock until he can't remember his own name is always gorgeous for Arthur, but he does eventually get bored of it, and flips them both over on the bed without a moments warning. Merlin underneath him with his legs spread wide for Arthur, head thrown back in bliss as Arthur thrusts roughly into him... another gorgeous sight. Equally gorgeous is the knowledge that Merlin will be feeling tonight for days to come.
Arthur comes after Merlin this time, enjoying fucking Merlin through that over-sensitised stage when his thrusts must bring at least as much pain as pleasure, and he pulls out this time, pulling Merlin's legs open even further and bending his knees to get a better look at his arse. His hole is fucked open, come oozing out lazily, and Arthur smiles, satisfied. That's exactly how he likes to keep Merlin – open and wet for him, because of him.
Merlin smiles at him again, at his most amenable at this stage, even though his eyelids are drooping and he's so sated he looks half-asleep. "Everything you wanted?" he asks, because he really does know Arthur better than anyone else, understands every one of Arthur's forbidden desires and gives into them for him, and yes, this – all of it – is exactly what Arthur wants. Not just Merlin's arse, opened by his cock and oozing with his come, but this whole thing they have going on.
He leans up and kisses him wetly on the mouth. "Yes." He says, quietly. "Perfect." They lie next to each other in silence for a few moments, then Arthur speaks up again, "You know, you won't be able to walk tomorrow," he can't quite keep the satisfaction out of his voice.
"Walk?" Merlin's own voice is sleepy but amused, "I don't think I'll be able to sit up." Arthur smiles, and Merlin's hand flaps ineffectually at him in what's supposed to a smack, but ends up as an affectionate sort of pat. He leaves his hand just resting on Arthur's hip, warm and affectionate. "And you love it, you bastard."
Arthur rests his hand on top of Merlin's at his hip. "I do." He agrees softly. They say nothing more, but somehow this thing – all of it – is just perfect.