Merlin staggers home at least once a week, wearing bits and pieces of Arthur's oldest armour, sweat making his hair stick to his forehead, covered in mud and bruises. He can't figure out why Arthur persists in this—Merlin's not improving with the sword at all, at least as far as he can tell from a rough estimate of how many times Arthur knocks him on his arse each session. And it surely can't be for Arthur's benefit, since Merlin is fairly sure that Arthur spends more time laughing at him and hauling him to his feet again than he spends actually practicing (which might have something to do with his lack of improvement, now that he thinks of it…). Besides which, Arthur leads the knights of Camelot, the best fighters in all of Albion. If he really wanted to practice, surely he'd rather do it with them than with Merlin of all people?
Merlin groans and lets the armour fall to the floor—he'll deal with it in the morning—smiling wanly in exchange for Gaius' assessing look.
"I'll get you some ointment." He says, and Merlin mumbles his thanks through another groan as he bends the wrong way getting his shirt over his head. He can feel the blue and purple of his skin, and Gaius' obvious wince when he re-enters the room does nothing to reassure Merlin that he might be mistaken. The bruises are a rainbow painted on pale skin, black and blue and purple marks overlaid on the ugly green and yellow and brown of the older ones that had finally (almost) faded. Gaius helps him with the ointment, applying it gently to the massive knot of tension and pain that is Merlin's back, then makes himself scarce so Merlin can handle the rest without an audience and get some rest.
Merlin knows that he's going to hurt in the morning—Gaius' ointment can only do so much—and that he'll have to suffer the indignity of Arthur's smirk every time he winces, and he spares a moment to wonder why he doesn't just refuse, the next time Arthur tries to drag him out to the practice yards. He's the prince, of course, but it would hardly be the first time Merlin had stood up to Arthur in spite of it.
The thought floats away with the smoke from the candle as Merlin blows it out and settles himself to sleep on his least-bruised side. His blankets are scratchy, and when they rub against his tenderized skin it's something quite a lot like pain.
Arthur doesn't see the bruises, but he knows where they are because he put them there. He knows where they are because he sees every time Merlin winces, hissing at the tug of sore muscle and flesh. He sees Merlin's graceful fingers clench; he sees Merlin catch his lower lip, soft and pink, between his teeth. He sees how Merlin glances at him—to see if he noticed? to blame him (justly) for the hurts that are (it's true) entirely his fault? or is he seeking comfort?—and Arthur can't help the way his lips curve, not when his smirk brightens both Merlin's cheeks and eyes at the same time, though Arthur knows—thinks he knows, hopes he knows—that they brighten for different reasons.
The bruises will fade in a week or so, and Arthur will know because Merlin won't whimper when he helps with Arthur's armour, and his lip won't be slightly swollen anymore.
So Arthur will take him out for more practice.
It took Merlin far too long to catch on, really, but Arthur had always told him he was an idiot, and he supposes in this one case it's possibly true. Arthur still knocks him on his arse every once in a while, but now he soothes the aches with wet, open-mouthed kisses that sting in just the right way. In between times, Merlin's bruises come shaped like Arthur's fingers on his hips, his wrists; like Arthur's mouth and teeth on his collarbone, his shoulders, his chest, stomach, buttocks, thighs.
Arthur's sheets aren't scratchy, and when they slide over those bruises… it's nothing like pain at all.
Arthur still smirks, whenever Merlin politely declines every invitation for him to sit down, and Merlin's cheeks still turn red every time, but his eyes go soft, now, rather than sharp and flint-like, and Arthur finds he prefers it that way.
He keeps making Merlin practice with him, though. Try as Arthur might, Merlin is still rubbish with a sword.