A/N: A response to this prompt on kinkme_merlin: Arthur/Merlin, wrists tied to ankles. I don't even know how this ended up being not porn, but there you go.
"Are you...okay...sire?" Kay asked as he and Arthur handed off their horses to the stableboys.
Grinning, despite knowing that was what was causing the concern, Arthur said, "I'm perfectly fine, Kay. Have a good evening."
Without waiting for a reply, he turned on his heel and quickly slipped around a corner and towards his chambers, eagerness in his step as he reached there.
He almost banged the door open, then remembered how long he was gone and what state Merlin was likely be in, and instead opted to open the door as quietly as possible, slipping in and shutting it with equal stealth, smiling as he realized he was right.
On his bed, the covers tented dramatically upwards, over Merlin's body, his head poking out from under them, his body propped up against the mounds of pillows by the headboard.
He softly set the sword on the fur rug by the fireplace, which was followed quickly with his chain mail and other weapons, until he was just in his breeches and a light shift, before stalking over and gently pulling back the covers to bare the sleeping warlock.
Laying back, slightly, Merlin's legs were spread wide open, being held there with Merlin's wrists tied to his ankles, his knees on the outside of his elbows, falling open in his sleep.
He was half-hard by the sight already
Making sure to lean slowly and carefully so's not to wake him, Arthur lowered his face and gaze to inspect the bindings, looking for something in particular.
Against Merlin's skin, and by the knots, were a few fine golden hairs - his own. And the fact that they were still there proved Merlin never untied the bonds even once during the whole day Arthur was gone.
Fully hard, Arthur now fought to keep from coming on the spot at the thought of Merlin being trapped here all day, waiting for Arthur, open and wet and wanting. Smirking, Arthur quickly and quietly finished undressing himself, setting the clothes with his chainmail and sword, and he stood over Merlin for a few moments, just drinking in the sight and imprinting the memory for future wanks, before grinning.
Raising his palm, he put in almost all the force he could muster as he brought it down on Merlin's arse.
"Aih!" Merlin yelped as he woke, startled and in a touch of pain. Arthur only barely held back. Merlin flailed for a moment as his hands jerked in all directions, bringing his legs with him. This only ended with Merlin in a heap on his side, glaring at Arthur, and flushing as his eyes kept flicking all over his patrol-toned body.
"Hello, Merlin," Arthur said, sitting on the edge of the bed. "I see you've been good."
"You mean I remained all trussed up for your strange and sadistic desires?" Merlin shot back.
"Oh, don't pretend you don't enjoy it," Arthur said, rolling his eyes. "I happen to know for a fact you never untied yourself. You stayed here wet and waiting for me...wanting me to come back. All. Day. Long."
Merlin's pupils blew wide open and he swallowed as he said, "For a fact?"
"For a fact," Arthur intoned, growling low in his throat in arousal as he climbed onto the bed. "And for that - you will be...rewarded."
Merlin didn't seem to mind being trussed up so much after that.