Summary: The devilry's in the details.
"Merlin," Arthur said, voice dry and arch from behind the screen, "I have the faintest suspicion that you've done something wrong."
He stepped out into the room, and Merlin almost choked on his tongue.
A moment ago, Arthur had been wearing his crimson suede coat (and complaining about a spot on the sleeve), and now...
Merlin blinked hard, but nothing changed. Well, Arthur hadn't changed. Clothing--he hadn't changed clothing. His clothing had changed. But it wasn't changing back.
"It is rather striking, isn't it," Arthur said mildly, and Merlin swallowed and nodded agreement.
Arthur was now decked out in a floor-length gown--the crimson had darkened to a rich burgundy, and now it was all tapered sleeves and swishy skirt and clinging velvet, and Merlin wasn't sure what not to look at first. He looked back down at the spellbook he'd been studying at the table, and then back up at Arthur. "That's not right."
"Astute as ever." Arthur ran a hand down the velvet over his stomach, his silver ring glinting against the burgundy. Merlin followed it, then shifted his attention to the pointed sleeve draping the back of Arthur's hand, and then decided he shouldn't be watching any of it, though his decision didn't seem to change anything, because he couldn't look away. Arthur tilted his head and murmured, "I don't see what Morgana's always grousing about. It's not nearly as uncomfortable as she makes out."
"Well, possibly there's supposed to be..." Merlin gestured helplessly in the direction of Arthur's well-muscled and not remotely feminine chest, nicely set off by the wide, gracefully scooped neckline. "Underthings."
Arthur glanced down, hooked a finger into the neckline and pulled it away from his body, raising his eyebrows. "Mmm. Good point."
"Probably difficult to fight in, too," Merlin said, and that was good, focusing on the practicalities. Arthur frowned, and took an experimental step-turn forward, one that Merlin recognized from the training field.
The skirt flared out, then swirled around his ankles. "You're right, it's completely unworkable," Arthur said, chuckling, and Merlin nodded, rubbing at his forehead. He'd never thought he would actually prefer a yelling Arthur, but this one was far too calm, and right now Merlin could really use someone to be completely astonished with.
Astonishment would give him something much safer to concentrate on.
Arthur gathered the fabric at his thighs, twitching the hem upwards (the hem of his dress, gibbered the part of Merlin that had not already given up and gone dumb), and peered down at a pair of matching burgundy slippers.
Merlin blinked at them. "I'm not sure I've ever seen a noblewoman's shoes before. All those skirts and things," he admitted. "Er, not that you're a noblewoman. Sire."
Arthur smirked at him. "They seem fitting." Merlin widened his eyes, and Arthur added, "Morgana used to throw hers at me when we were younger. Completely undeserved, of course."
"No doubt," Merlin said, and swallowed hard. His errant magic seemed to have sucked all the moisture from the room.
The folds of the dress soaked up some of the light and threw the rest out in brilliant garnet tones. The effect was mesmerizing, and Merlin didn't quite realize how close Arthur had gotten until he stopped in front of Merlin, waving his fingers in front of Merlin's eyes. "You are planning to change it back at some point?" Arthur raised a hand and teased out his bangs into a delicate spread, pursing his lips in an exaggerated pout. "I could wear this out, but I'm afraid I don't have the jewelry to match."
"Of course," Merlin said, ducking his head. It was, after all, entirely his fault, though he wasn't sure exactly what he'd done. Put Arthur in a dress had decidedly not been on the page he'd been studying.
He found himself staring at the barest dips of fabric around Arthur's ribs. "What's it feel like?" he asked, faintly.
Arthur took his measure, eyes luminous in contrast with the gown. "Come and see for yourself."
Holding out a hand, Arthur beckoned him forward, and Merlin closed the space between them, almost against his will. Catching his wrist, Arthur brought Merlin's palm flush against his hip.
Under his hand, the fabric was warm with the heat of Arthur's body.
Merlin closed his eyes for a moment, then slid his hand down, and the material ruffled, as if protesting his movement. Switching directions, the velvet was smooth, soft, enticing, almost like the fur lining Arthur's favorite chair.
Merlin slid his hand past Arthur's waist, up his side, along the seam to his shoulder. He met Arthur's eyes, and they were smoldering.
Arthur lunged forward to kiss him, and Merlin met him eagerly. He let Arthur press him back against the bed, the velvet whispering between them, spilling the last of their secrets.
Arthur's hand slid gently up Merlin's bent back, warning him that Arthur was awake before his voice rumbled up from the pillows. "So, is there something I should know about your sartorial designs on me?"
Merlin turned his head to see Arthur giving him a look of sleepy amusement. "You mean, do I want to see you in a dress?"
Arthur lifted his head enough to send a pointed glance to the tangle of fabric at the foot of the bed. Merlin grinned and shook his head. "No, not really. I was trying to get out that stain you were on about, and I..." He waved at the spellbook, now lying open in front of him on the bed. "I picked the wrong word."
"The wrong word?" Arthur sat up, too, the sheets pooling around his waist. "If that's the kind of thing that happens when you get a single word wrong, don't ever do that to anything I happen to be wearing ever again. I shudder to think what might happen."
"Oh, I don't know," Merlin said, smiling brightly, rubbing his knee against Arthur's bare thigh, under the covers. "I think it worked out moderately well, don't you?"
Arthur's answering scowl had to be purely theatrical, because it only lasted until both book and dress had been knocked unceremoniously to the floor.