Arthur waits years, but Merlin doesn't confess.
Merlin seems to think Arthur is oblivious to what he does, but Arthur has seen him raise protections and rain fire, has seen him kill - Arthur knows what Merlin is capable of.
The knowledge makes him shiver. Because Merlin undresses him with the same hands he uses to handle the worst magic, the same hands that have pulled the life from Arthur's enemies. Every finger is a weapon, and Arthur can count them as they glide over his skin.
Every day Arthur swears he'll confront Merlin, but it takes escalations of hostility on the northern borders, it takes a bloody skirmish and fifty of Arthur's men dead before he's angry enough and stupid enough to say something.
He can't take the lies any more, he can't stand Merlin squiring for him as if he were just another dimwitted servant and Arthur just the noble he works for, as if Merlin doesn't particularly care for the job or his overlord. As if this is something normal. Here and now, it will out; here and now, Arthur will take the truth that is his due, will have Merlin's fealty or his hide, one or the other. Quiet and careful and predatory, Arthur draws his dagger.
Merlin is on his knees, unbuckling Arthur's fauld from around his waist, when Arthur presses the blade to his throat. The armour clatters to the floor. 'I know what you are,' Arthur hisses, and Merlin freezes. 'How long were you planning to wait before you killed me?' Arthur asks, drawing the blade tighter, forcing Merlin to move closer to avoid cutting himself. 'What else were you planning?'
'Nothing,' Merlin whispers, looking up along the length of Arthur's body. 'I'm your servant.'
'Arthur, please -' Arthur jerks the dagger, just a little, and it kisses Merlin's skin, and Merlin stops talking, stops breathing - the edge takes a tiny slice from his throat and he stays there. He could get away, Arthur knows that. Merlin doesn't need to allow this. Merlin doesn't need to let Arthur shed his blood drop by reluctant, oozing drop, but he does. He gasps a shallow breath and his eyes flutter, obscene like this arouses him, and maybe it does.
He kneels at Arthur's feet, all that deadly power held tight and whip-sharp in his thin frame, and the way he looks up makes Arthur suddenly ache to force him, to prove he can, because would it be force? If Merlin could kill him easy as a thought?
'How many lives did you take today?' Arthur rasps.
Merlin closes his eyes, swallows, and his Adam's apple touches Arthur's blade, collects another tiny slice. 'I - I didn't count.' He mutters it. He's ashamed. Then, 'Enough of them,' he says, and looks back up with a tiny hint of defiance. 'You're still alive, aren't you?'
'Was that your aim?' Arthur catches Merlin's jaw in his free hand, but resists the urge to reel him in. 'To keep me alive? You killed all those men to save my life?'
'Yes.' Merlin's voice is low and urgent, as if this is the most important part. 'Always.'
'You'd use magic for me?'
'Arthur,' says Merlin, and he presses one hand to Arthur's knee and he swallows again, scrapes himself again, and Arthur can feel the trembling of him through the knife, 'For you, I'd do anything.'
And he means it, he does, kneeling there perfect and frightening and submissive and totally unrepentant, making servitude look like a virtue, bleeding for Arthur because Arthur made it happen - he's so vulnerable. He's so powerful.
Arthur has never wanted anything so badly in his life.
'Would you do something you hated, for me?' Arthur asks.
'Yes,' Merlin says, without hesitation.
'Would you do something you knew was wrong?' Arthur asks, and draws the blade a little higher, bringing Merlin a little closer, and he has to see how Arthur's straining against his heavy breeches, all sweat-and-battle-stained, he has to see how hard Arthur is for this, for him.
'Yes,' he says again.
'Suck me,' Arthur says, and doesn't move his knife, and Merlin tugs down the breeches in his way and pushes close, close enough to lick, close enough to be cut. His lips trace the shape of Arthur's cock slowly, as if he's wanted this, as if he's thought of it before.
Arthur pulls him in with his hand splayed, fingers cradling the curve of Merlin's skull, the dagger now trapped between them with the blade against the delicate skin of Merlin's throat, the hilt pushing hard against his own thigh. 'Suck me, I said,' Arthur almost growls, and feels the silk-rasp of Merlin's eyelashes as his eyes close before wet, thick heat envelops him.
It's tight, for Merlin's mouth can't open far enough with the knife in place, can't loosen, can't let his tongue work, and Arthur draws himself out a little, holding Merlin in place with hand and weapon and willpower, and then thrusts back in - sure, hard strokes, thorough and pure of artifice, as if he were taking a maid over his table in his chambers. Merlin holds still for him, holds wet and breathlessly eager and keeps his teeth covered, his jaw loose, and he swore that he would do anything for Arthur, but this is such beautiful acquiescence.
Merlin kills and Merlin launders and Merlin sucks cock because Merlin decides he wants to, and not for any other reason.
Arthur's hips stutter free when he realises it, and Merlin opens his eyes and looks up, all burning emotions that Arthur can't read properly. His lips part as Arthur withdraws. They're pink-red, and Merlin licks to catch the wetness that clings to them, still staring. Arthur's trying so hard to keep control of himself, thinking oh God, what have I done?, and then Merlin reaches up one hand, the movement making the blade that's slipped to his collarbones glint in the candle-light, and wraps his fingers around Arthur's cock. He strokes once, strokes twice, and Arthur tries to resist. And then he leans into the blade so that the quillon presses white and the blade presses a thin line of red into his throat, and he presses a kiss just below the head of Arthur's cock, and Arthur cannot control himself any more.
When he opens his eyes again, it's because he's dropped the dagger, slippery with his mess, to the floor. Merlin's eyelashes are flecked with wet, his face is streaked from the shadows under his eyes to the hinge of his jaw, across his hungry mouth, and his throat is bleeding from shallow, stupid slices, and his hands are between his thighs, and he's shaking, panting, pleading with his eyes and with his posture, just aching to be pushed backwards, aching to be had.
Arthur overbalances him the way he wants and cradles him, one arm around his shoulders and the other knocking his hands away to take hold of the cock that peeks from the laces of his shameful, stained breeches. Merlin's blood is smearing his hands, and Arthur wishes he had never drawn that knife.
'Do you truly think this is wrong?' Merlin gasps, licking his lips, pushing his face into the shoulder of Arthur's gambeson, staining it. It doesn't matter - no more blood can hurt and as for the other fluid, well, Arthur will hardly be the first man to smirch the trappings of war in such a way. 'Did you truly think I'd hate this?'
Arthur grits his teeth and licks his hand, tasting metal. Slicker is better and Merlin keens for it, arching into Arthur's hold. 'You pushed me and you cut me, Arthur, but you haven't hurt me and you couldn't force me -'
'Come, damn you,' Arthur mutters, wanting an end to the talking and wanting something of Merlin's to wash away the blood with. 'Come for me, Merlin, come on -' Merlin does as he's told, Merlin gives himself over to it, and to Arthur.
'If you need it, I'll give it to you,' Merlin moans as he comes down, as his spine unknots in the afterglow. 'If you want it, I want it too,' he says in a voice that's scratched, and he paddles his fingers in the streaks on his skin, and rubs them into Arthur's clothes. 'There's nothing you can do to me that I won't take, Arthur. Nothing.'
And he means it, every word of it. He's offering Arthur everything - everything he is. His loyalty, his body.
Bowed over Merlin's body and painted in his colours, Arthur sees Merlin's beautiful, faithful treason at last.