Title: The Burning Star
Rating: PG-13
Summary: Sorcerers are dying in Camelot. The Doctor happens to be in the right place at the right time, as he so often is.
Warnings: Awful title? Poorly conceived Baddie Of The Week? No spoilers per se.
Notes: Written for the Merlin/Arthur FQF, Round 2, Challenge 31: Doctor Who crossover. The Doctor lands in Camelot and somehow winds up getting Merlin and Arthur to realize their true feelings. I fulfilled this in the vaguest way possible, if at all. *hangs head* Also, I am one of those terrible Johnny-come-lately Whofans who hasn't seen the old seasons; please adjust your expectations as necessary.

****

Arthur and Merlin trudge towards the city at nightfall, Arthur frustrated by a fruitless hunt and Merlin bearing the brunt of his annoyance. There's something tense in the air that's been making the animals jumpy, and every possible prey would dart into the undergrowth without the slightest provocation.

"It's not my fault that you were being too loud," Merlin says anyway.

"I was being too loud? You were probably trying to scare them off on purpose. Don't think I don't see you wincing..."

"Still not my fault," Merlin says briskly. They both pause to watch a shooting star cross the city in a dramatic golden streak, as bright and brilliant as daylight.

"Beautiful," Merlin murmurs.

"Don't be such a girl. Come on."

Merlin follows him, so close in the darkness that their arms bump together, but Arthur doesn't bother to widen the gap.

****

A week later, in a little bakery in Camelot, Telyn is dying.

Telyn was a sorceress, once. She brought her family through famine and sickness, she helped her friends and eased their pains and healed their wounds. She had four children, and when her youngest showed the gift she was overjoyed. She loved her magic, cherished it as one would any blessing, and she never once thought to use it for ill.

Then came the Great Purge, and she was no longer a sorceress. Her husband begged her to leave Camelot, to run as far as she could from the King's rage, but she refused to go from the place she loved. Her friends kept her secret, and years later she is simply a baker's wife, living quietly in the lower city.

There are many like her. The villages, the city - even the castle itself is ripe with magic kept secret. The Purge rolled through the kingdom like a wall of fire, true enough, but not everything was burned.

Now, in the dead of a cold night, Telyn begs for her life, not for her own sake but for the sake of the children and grandchildren who rely on her.

"They will live on after you are dead," it says, speaking with her own voice. "Just as I am surviving, when she is gone."

Then it kills her. And then it moves on, a plague moving silently through Camelot.

*****

He steps out of the TARDIS, takes a gulp of cold air and grins like a little boy, his teeth gleaming in the sunlight.

"Camelot! It's magnificent, Donna! Come and have a look." He pauses. "Donna?"

Donna is still, irritable and arms folded, on the threshold.

"Camelot," she says. "Yeah, right. Where're we going next, Hogwarts?"

"Don't be daft. Closed ages ago. Coming?"

He starts walking, and Donna realises after a few hundred yards that she's following him. Still incredulous, arms still folded. But wherever they are (not Camelot), she has to admit it does look magnificent: a great white castle spread across the horizon, banners waving in the wind, silver smoke rising from the chimneys.

"Camelot," she says finally, "is fictional. Fick-shun-al. Not real. Yeah?"

"You mean it's not in the history books," the Doctor says cheerfully. He's found a road leading into the city - of course he has - and they make their merry way. It seems awfully quiet.

"No, obviously - "

"Well, sometimes, the only difference between a thing being history or fiction is what bookshelf you put it on." Another blinding grin, like he thinks he's won the argument, and Donna rolls her eyes and stumbles after him.

*****

Gaius is grey, looking his age, as he pulls the sheet back up over her face.

"She was the seventh this month," he says quietly, and Uther simply nods.

"Do you know how she died? How any of them died?"

"There are no wounds, sire, nor anything that would suggest illness. I have spoken to her family; they said nothing was amiss. I will test for poisons, but..."

"But it is likely that she died without cause, like the others." Uther's brows are low, his mouth a thin tense line. "Tell me what I am thinking, Gaius."

The physician sighs and hangs his head. He hates it, loathes it, when the king is probably right to accuse his favourite scapegoat. "I have no reason to believe this is the work of a sorcerer. Not yet."

"Then you are giving him exactly what he wants. All sorcerers...they like nothing better to destroy everything we hold dear and then slither away, unpunished, like snakes in long grass. If you have no reason, then I suggest you find it. Soon."

Merlin hangs back at the steps from his room, watching Uther leave in a flurry of red cloak and righteous rage. He stares at the shape beneath the blanket. "Is it sorcery?"

"It would seem that way, yes." Gaius rubs the bridge of his nose. "But it is quite unlike any sorcery I have ever seen. It's too pure, too powerful. Take care, Merlin."

*****

"Lord John Smith, of Mercia," and the Doctor holds up his psychic paper until Uther's furrowed brow smooths out. "And this is Donna."

"Lord John," Uther says. "Our first Mercian guest since the treaty; you and your...wife are welcome at Camelot."

Donna chokes on the air. "His wife? Him?"

The Doctor blinks. "Oh, she's not my - I mean, we aren't - she's my maidservant," he says, and gives Donna a quick warning look before she can throw an apoplectic fit. He wonders if 'maidservant' is a step up or a step down from 'the plucky young woman who helps me out' and suspects the latter.

"I see. Forgive me." Uther leans forward, eyes narrow. "I am told you arrived without horses, luggage, any sort of retinue...quite unusual, for a man of standing."

"Ah. Well. Yes. You see, we were..."

"Mugged," Donna says, and the Doctor gives her a delighted little quirk of his lips.

"Exactly. Just inside the borders of your lovely kingdom, huge group of bandits, really quite traumatic, took everything but the shirts off our backs. I sent the others home, but Donna insisted on tagging along. She's amazing, really."

"Mm." Uther gives Donna a long, considering look, but she's busy looking over his shoulder at some unfeasibly gorgeous chunk of blond maleness who's hanging around Uther's left hand. A few paces behind him is a taller, thinner man/boy, with a dark nest of hair and ridiculous ears.

Uther clears his throat. "Men will be sent to look for these raiders on the border. In the meantime, we will find accommodation for you within the castle. Arthur?"

Blond McSexy steps forward, and Donna's face twitches.

"Arthur?" she whispers urgently to the Doctor. "The Arthur? Knights of the Round Table and Excalibur and all that?"

"Yep." He smiles and bows to the Crown Prince, hauling Donna down with him despite the indignant noise she makes. "It's a great honour to make your acquaintance, sire."

"Same," Arthur says, in a tone that suggests it isn't. He leads them out of the room, away from his father and other prying eyes. "I'll have Merlin--"

"Merlin?" Donna says, before she can be stopped.

"You know him?" Arthur says, nonplussed. "You have my deepest sympathies. He'll find you some rooms...and see about some clothes," he adds, reviewing the Doctor's scruffy suit and Donna's jeans with palpable disbelief.

"What?" The Doctor tugs at his lapels. "This? All the rage in Mercia. Can't get enough of it."

****

The eighth victim worked in the castle's laundry. The king, unnerved by a killer on his very doorstep, orders the guard doubled and a curfew enforced. Merlin helps Gaius where he can, setting up equipment and preparing herbs, and he tiptoes around the physician because he looks older and more fragile than ever.

"There is something I need to tell you, Merlin, that I could not tell you with Uther close by," he says.

Merlin's hand stills on the pestle. "Oh?"

"I...have been allowing the king to believe that these deaths are unrelated, and in doing that I have as good as lied to him." He exhales in a ragged sigh. "They are - were - sorcerers. All of them."

Merlin says nothing for a long few moments, staring intently at the body on the bench. "All of them? But they stayed in Camelot, even when...?"

"I knew Telyn well," Gaius says. "I was one of many who would have had her leave, but she loved this place so much. Her family, her friends...they were all here, and so she stayed. Her loyalty to them was greater than her fear of the Purge - and it seems that they felt the same way about her. Even in death they have not betrayed her."

Merlin nods, feeling an odd combination of guilt and respect for this woman he never knew. He gestures tinily to the body. "And Bowen?"

"Was her youngest son," Gaius murmurs.

"What are we going to do?" Merlin asks. He asks this, and not 'shall we tell the King?', because they both know how Uther Pendragon will respond to a murderer who targets only sorcerers.

"I will try to identify whatever man or beast is doing this - is capable of killing in this way," Gaius says. "And you, Merlin - "

"Merlin!"

They both look up, doing their best respective sorcery, what sorcery? expressions as Arthur tromps in. He looks unhappy, and the normality of that is rather soothing by Merlin's standards.

He tries for innocence. "What?"

"Merlin."

"What's the matter, sire?"

"Better. Marginally." Arthur sets his weight on one hip. "Did you find a suite for our guests?"

"Yeah, I put them in the north wing - "

"Good. Keep an eye on them, I refuse to believe anyone in Mercia dresses like that. And Merlin?"

"Yeah?"

"There's a killer running loose, and I haven't the time to train up another manservant. Don't go wandering off by yourself. If Gaius wants you to go collecting herbs or mushrooms or whatever it is he needs..." Arthur realises his subject is standing right next to him; he clears his throat. "Gaius, Merlin isn't to leave the city for any reason."

"As you command, sire," Gaius says, and Merlin looks bemused by the idea that he's been in any way trained.

****

In a little-used wing of the castle, in a room where he knows he won't be disturbed, Sir Aled is passing a pleasant evening with Willow, a girl from the castle laundry. Or rather, he was. Because Willow is not Willow any more, but a puppet with her semblance, speaking in a strange metallic voice which is her own and not her own.

"Leave," Willow grates. "Go from this place. Let me feed."

Aled stares, his body rigid, his hand going to his sword - he came here half-armoured in his eagerness to see her. "There is a monster in Camelot," he says, forcing his voice level. "This is...you are..."

"A monster? This is my nature."

Her body twists, and Willow whines in her own voice, her face tight and clenched with pain. Aled's grip tightens on the hilt of his sword.

"Stop it! You're killing her!"

"Do you love her?"

Aled hesitates, for too long, and they both know that he is not steeling himself for honesty but arranging an answer that might satisfy the monster.

"You care nothing for this one, as they cared nothing for her. Let me feed."

"If she is to die..." Aled draws his sword, slowly, watching the way her face pales and her breath stutters. "Better at my hand than yours, monster. Better, if you are to perish with her."

****

At dinner that night - not a feast, but a pretty lavish affair in honour of the visitors - the Doctor insists that Donna is his companion in all things and hence will be eating with him, not waiting on him. Uther twitches and mutters about propriety and since when did Mercian nobility behave in this way, but eventually lets it happen. Instantly there's a ripple of rumour that Donna must be Lord John's bedmate which Donna. Pointedly. Ignores.

"I'd just let myself find it flattering, if I were you," Morgana says. Morgana (Morgana! Donna echoes to herself, although she's finally managing to keep quiet about it) is seated next to her; she's pale, almost translucent, and intimidatingly beautiful. "He's not altogether unattractive."

"Pssh, if you like that sort of thing - which I don't. He turns sideways and disappears. What's to like? Literally nothing."

"Nothing wrong with skinny men," says Gwen (bloody Guinevere) from over Morgana's shoulder, and Morgana stares at her with a mischievous smile until she blushes.

"No accounting for taste. What about those two, then?" Donna nods at Arthur and Merlin. Arthur is whispering something to Merlin, who's leant forward to top up his goblet; Merlin whispers something back, and Arthur scowls at him but can't disguise a smirk when he looks away. Donna doesn't know what it's called in Camelot but in Chiswick she'd call it flirting.

"What about them?" Gwen asks.

"Are they...y'know." She decides not to try her luck with the Nationally Accepted Handsign for 'having it off'. "Friends."

Morgana makes an extraordinary little noise into her wine, but she comes up looking oddly thoughtful.

"Well," Gwen tries, "I don't think either of them would see it that way, but with the things they've done for each other...I mean, I'd call them friends..."

"Um, no. I mean. Friends."

Gwen looks bemused, right up until Morgana beckons her down and whispers something, which prompts a squeaky "ohh!" and another blush.

"No!" she says on recovering. "I mean, um, of course not. They wouldn't. They're not...of course they aren't. They can barely have a civil conversation, never mind...um...whatever gave you that idea?"

"I'm sure I can't imagine," Morgana says easily.

"To be honest, I...I always thought he liked you, my lady."

"And when was the last time he had a 'civil conversation' with me?"

"Hmm. Oh. Oh, I see." Gwen looks across at Merlin and Arthur with revelation in her eyes.

"Off your rockers, all of you," Donna mutters into her cup.

****

"Please forgive me, sire," Sir Aled croaks, his voice breaking with guilt. "She was dying, the monster was inside her...I...I thought it was over, but then..."

Arthur pats his shoulder. "Don't worry. It is enough for me that you survived. Just rest."

Aled nods, drifting back into unconsciousness a few moments later, and Arthur looks across the room. Gaius is shoulder-to-shoulder with their Mercian guest - 'the king's physician as it happens, helpful coincidence, I know, let's have a look at this then' - as they review a spread of ancient tomes, speaking in low voices about sorcery and knights and public panic. His maid, who seems about as obedient and competent as - well, Merlin - is poking around Gaius' equipment and basically looking useless.

"Gaius, Lord John," Arthur says. "My father and I are growing quite anxious. A killer that leaves no trace, that has no home...is there any explanation for this? At all?"

Gaius shakes his head. "Not yet, sire, but it has become clear that this is the work of a sorcerer. Sir Aled told me that this - whatever it is - it seemed to possess Willow, and they had words before she was killed."

"What did it say?"

"Nothing he understood," Gaius says, looking back to the books.

"Keep me informed," Arthur says, then turns - to barely avoid being knocked out by a solid oak door. He grabs it before it can break his nose and glares at Merlin, hanging at the threshold with an armful of herbs.

Then he grabs Merlin by one large, eminently grabbable ear. "You utter dunce, what did I tell you about - "

"I know! They're from the kitchen. Dried. I didn't leave the castle," Merlin says, squirming, waving a dehydrated sprig of rosemary until Arthur is forced to concede.

"Fine. Fine. Just don't go getting any ideas, because I can tell you right now that they'll be ridiculous." Then he stomps off, shoulders tense and head sunk.

Gaius watches him go. "I'm afraid I have no choice but to tell the King what I know," he says, reluctantly, to Merlin. "An attack on a knight...this beast's movements will be under much greater scrutiny from now on."

****

Uther rests one elbow on the armrest of his throne and leans into it, looking the very picture of regal calm and control. He isn't quite smiling but he's not far off it, either.

"A monster that kills sorcerers and no-one else?"

"It seems that way, sire. The tests I have performed indicate that all the victims to have been killed were..." Of course, there are no such tests, and Gaius is pretty much betting his life on Uther not knowing that.

Which, it turns out, he doesn't. "So Sir Aled is not...?"

"No, sire."

"Good. Good. It would be highly disturbing to find a sorcerer among my most trusted ranks. Arthur."

"Yes, sire."

"Have the search called off and the curfew relieved. I don't think Camelot should be troubled by this 'threat' any longer."

Arthur's mouth twitches. "Sire, our people are dying - "

"Sorcerers are dying," Uther says, his voice hardening, "and they are not our people. They are a menace. Should this monster appear to me, I shall do nothing, save grant it the freedom of Camelot."

"And what if it tires of killing sorcerers?" Arthur says, low and urgent. "What then?"

Uther doesn't straighten, but his body freezes in place, his brows lowering. "I have said my last word on this issue, Arthur. Go and make my orders known."

"Father - "

"My last word."

****

"Arthur?" Merlin is so annoyed that he abandons sire and my lord and even prat so as best to get the Prince's attention. It's almost dark, he's finished his duties for the day and one of his room's favourite features is missing:

"My bed's gone. Gaius says you had men come to get it. Why would you take my bed? I like that bed, it's my favourite thing for - for - "

"Sleeping?"

"Well, yeah," Merlin says, caught short by the fact that his bed is at his feet, right next to Arthur's bed, where Arthur is lying and reading a book and looking irritated. "Why is my bed here?"

"Still to sleep in, I expect, since you are absolutely not to bring any girls in here." Arthur pauses. "Unless they're for me, in which case the effort is appreciated but unnecessary."

"What - I don't - I mean, that's not - why did you move my bed?"

"The monster."

"The monster that only kills sorcerers," Merlin says, forcing his heart back down his throat.

"Mmm. Here's the way I see it, Merlin. You're an idiot of tremendous proportions."

"If you say so."

"And anyone stupid enough to continue as a sorcerer, in Camelot, is also an idiot."

"I...well..."

"Hence, this monster might well take a look at you and see a likely victim, not simply a dense manservant. And fleet-footed as I am, I likely won't be in time to come and save your useless arse should you be dying in your own room."

"That's...." Words fail. "Sweet?"

"It's simple logic, Merlin. And since my father wants no part in killing, hunting or even deterring the beast, you may be here for some time into the future."

Merlin worries at his lip. "He doesn't want to kill it?"

"He practically wants to knight it, whoever or whatever it is." Arthur closes his book with an authoritative snap. "Don't you have anything to do besides whining?"

****

Any last lingering hopes Donna had of a slightly surreal Medieval holiday vanishes, poof!, the moment the Doctor produces one of his ludicrous flashing-and-beeping devices. She's also disgusted - if not very surprised - to find that his bed is infinitely comfier than hers. She sits down heavily and flops backwards, watching the Doctor doing something to Flash'n'Beep with his sonic screwdriver.

"Oi. What's going on, then?" It hasn't escaped her notice, either, that his easy smile has gone and been replaced by something more determined.

"People are dying here," the Doctor says. "It'd be irresponsible not to find out why."

Donna thinks about the Pyrovile, the Sontarans, the Vashta Nerada, and she thinks it's a bloody miracle that anyone's still alive but then she realises it's not a bloody miracle, it's just...him.

She refuses to feel awed. "Yeah. So what's your beepy thing for?"

"Detects and tracks the use and movement of esoteric etheric energy," he says, and smiles again at Donna's dead-eyed stare. "Or: magic."

"It's never magic," Donna says flatly. "I know they've been going on about sorcery and whatever, but no. Don't even tell me: it's gonna be some alien who makes people think they're sorcerers, or some spaceman pretending to be a wizard, or some weird outer-space something-or-other that's giving people magic powers..." She looks at the Doctor, who's gone back to fiddling with Flash'n'Beep. It looks like a calculator mated with a hot-air brush. "Isn't it?"

"Nope. Real magic. Still exists on Earth, in your time, after a fashion - every so often you find a human who can use it, does use it, but nowhere near this scale. We're about twenty years late for the real age of magic in Camelot, but the air's alive with it, you can smell it, you can..." He flicks at the air with his tongue. "You can taste it." He looks delighted.

Donna decides that, out of all the disbeliefs she's had to suspend, this is pretty tame. She sits up. "So if there's all this magic, they can take care of themselves, can't they?"

The Doctor cocks his head. "Been looking that way so far. Problem is, there are species that don't use magic like they do on Earth."

"So how do they use it, then?"

He frowns. "They feed on it."

****

So Merlin is trying to sleep. Key word trying. It's strange, how frustrating sounds can become a lullaby over time: Merlin once hated the distant buzz of Gaius' snoring and the clatter of ill-fitted doors in their frames, but here and now it's just too quiet. The only thing he can hear is Arthur's breathing, deep and steady, seeming so close he can almost feel it on his skin.

Merlin throws himself left and right, twisting the blanket around his body. He's not dead yet, so that's just great. Getting some rest won't kill him. But, as noble (?) as Arthur's motives are, Merlin won't be best placed to protect himself if he's sleepwalking through his duties.

Merlin considers waking him up out of spite. But he doesn't have to.

"Can't sleep?"

"I'd be sleeping just fine if I was back in my own room," Merlin says. "Thanks again, by the way."

"I need to keep an eye on you. It's this, or your life being placed in mortal danger and me having to get you out of trouble yet again."

"I've never asked you to help, you know. I can look after myself."

"I'd give you an opportunity to prove it, Merlin, but I've more or less got you broken in now and replacing you would take too long. I don't go saving your arse on a weekly basis just for the pleasure of it."

"The pleasure of my arse?" Merlin blurts.

The pause that follows is dangerous. Arthur clears his throat thunderously.

"Go to sleep."

****

Come morning, Gaius' room has no Gaius in it, since apparently he's out making the rounds. What it does have is Lord John and his odd maid, which is strange at best and suspicious at worst. Merlin bites his lip; this is the only time, he realises, that he's likely to be alone with them. Something's been gnawing at his brain since he first saw them from behind Uther's throne, and this is the one chance he'll have to bring it up.

Deep breath. "My lord. Um. Lord John?"

He looks up and smiles, which is suspicious in and of itself, because nobody north of Gaius on the pecking order ever smiles at him. Except Arthur, sometimes, but Arthur is a case all of his own.

"Merlin! What can I do for you?"

"I..." Deep breath. "You told the King that you're Lord John Smith, from Mercia, and you showed him...something?"

The man blinks, makes a little 'ooh' shape with his mouth like something is immensely interesting, then digs in his pocket. "You mean this?"

"Yes. I think. Only...that's..." Merlin makes sure the door is still half-open for ease of escape. "That's blank. There's nothing there."

"Well, that is fascinating."

"It is?"

"I thought you were clever, the moment I saw you! Didn't I tell you, Donna? 'Ooh, he looks clever', I said, and here you are. Being clever." The man crosses the room, looking at Merlin like he's something completely unique - and like he recognises that about him, not that he's an idiot or a manservant or a disaster waiting to happen but that he's...what he is.

"But it's more than that. More than just clever. Isn't it?"

Merlin scrabbles for the door without daring to look at it. "I - I think I should go."

Before he's recognised her as a threat, Donna crosses the room and shuts the door with a resounding thump - but it's her master she glares at.

"Leave it out - the big mystery act? You look a right idiot," she says to him, hands on hips, then rounds on Merlin. "He's the Doctor, I'm Donna, but I s'pose you knew that already. Whatever's doing the rounds here, killing people? He's going to find it, and do summat about it, because...it's what he does, you can hardly stop him." Merlin notes a level of admiration under the derision, but decides not to mention it.

"We're going to do summat about it," the Doctor corrects her.

"As if. What'm I gonna do, notate it to death?"

"Doctor - Donna," Merlin says weakly. "So you're...not from Mercia."

"Nah, just a bit of fibbing to get a foot in the door - harmless, though," the Doctor says cheerfully. "Now. Been doing a bit of research - great stuff, all this, by the way, your Gaius has been doing more than his fair share of reading over the years, eh?" Then his smile fades. "But you're not going to find it. Not this thing killing sorcerers."

Merlin goes rigid. "Why not?"

"Because these books? All these monsters, all these myths, all this knowledge...it's as good a record of life on this world as you'll find anywhere, right now, but that's where you've gone wrong. Your murderer won't be in these books."

"Because...what, because the creature is from somewhere..."

"Beyond this world, your world?" The Doctor nods. "Yes."

Merlin wants to run and find Arthur, grab him and shake him, because here are two people demonstrably more ridiculous than he is. But he doesn't. For all that the Doctor is clearly insane and untrustworthy...Merlin knows he isn't lying.

"Do you know what it is?"

"I wasn't sure, until that business with your Sir Aled. You see, I've come across plenty of species attracted to magic, but most of them are less discriminating. They'll feed from any source, even if they prefer sorcerers - because magic is life, it's a form of energy, but it's something purer and more powerful than most humans will ever encounter."

Donna mutters something, it sounds like 'don't tell him stuff he'll know already', and Merlin slides back a step.

"So this is something that...eats magic," he says, uncertain. "So that's why it didn't...I mean, Sir Aled, it only..."

"It felt threatened, so it attacked him, fed from him, but to the Subsumate that's like - it's like drinking tainted water, or eating rotten food. It neutralised the threat but now it'll be hurt, it's been poisoned, the only way to - "

"Oi!" The Doctor looks at Donna like he's forgotten she's there. "Two steps back for the humans, if you don't mind. Subsu-what?"

"Oh! Sorry. Right, the Subsumate. Two species, the Subsumate and the Domisuma - no fixed home any more, was destroyed millennia ago, they tend to hitch rides when they can. They're symbiotic - one can barely survive without the other. The Subsumate has no physical form, it's a moving parasite - it hops between people and animals and what-have-you, feeding on their esoteri - sorry, their magic, but when they need a place to sleep or heal or reproduce, they go back to the Domisuma, which acts like a - a home, a base. The Domisuma can only feed on energy from the Subsumate, they have no way of taking it from just anything, and the Subsumate need a place to stay. So it's mutually beneficial."

Merlin stares at the Doctor, moving his mouth slowly although he can't make any sounds come out.

"He does that," Donna says, unapologetic. "All the bloody time."

Merlin suppresses the violent urge to run away screaming, then nods. Gaius has got him trained to absorb vast amounts of information at a time, even if he doesn't believe a word of it. "Okay. I think I've got all that."

The Doctor beams. "Said you were clever, didn't I?"

"Uh," Merlin says. "So, there's two things-that-aren't-from-this-world, here?"

"Nnnno. Just the one."

"But you just said..."

"We-ell, here's the thing. If there was a normal pairing here, there'd be no problem. All things going well, the Subsumate don't kill. They don't need to. Few sorcerers with the odd headache, otherwise you'd never notice. But when one's separated from its host, when there's nowhere for it to go, they have to gorge themselves just to stay alive. Without a Domisuma to return to, they just...fade away. They die. So this one keeps jumping between human hosts, gorging itself, but humans can't maintain a parasite of this power for long, so..." The Doctor nods towards the latest victim, still covered by a white cloth.

Merlin sits down heavily on a stool near Gaius' workbench. "Oh."

"I'm sorry. This is a lot to take in."

"You're not kidding." Merlin tugs at his hair. "Why are you telling me all this? Gaius would be better, or...actually, never mind, just Gaius," because Uther would do a lot of shouting and then have them all executed.

"I'm telling you, because like I was saying, the Subsumate has been poisoned," the Doctor says, and this time his voice is low and sincere. "The only chance it has of healing itself is to find a new host, one with extraordinary magical power, and to drain it dry."

"Oh," Merlin says, very quietly. He looks up. "But...how can it be killed?"

The Doctor makes a face like he'd rather it didn't come to that. "Sir Aled thought that killing the host body would destroy the Subsumate, but it doesn't work that way. Weapons are useless, magic is useless - you can try all the spells, all the enchantments you know and they'll just pass straight through. They're creatures of pure energy. The only thing that can hurt them is what they consume."

****

The day finds the Doctor and Donna in the castle corridors, trailing left and right at the whim of Flash'n'Beep. Everyone they come across seems more than ready to accept 'it's from Mercia' as an explanation for the oddity of it, although Mercia is a stone's throw away and no different from Camelot, at least not in any measurable way. The only one who bothers asking questions, Gwen en route to seeing Gaius, is eventually scared off by a rapid-fire barrage of technical jargon.

"It's still in the castle," the Doctor says, fiddling with something that looks far too much like the handle of a mixer tap. "It's been jumping between bodies, looking for its next meal...it seems to have settled recently, but I can't tell..."

There's a cry of shock, like a yelp, from somewhere behind them, and they both hear the maid calling out.

"Gaius...? Gaius-!"

The Doctor goes from stationary to sprint in no time at all.

****

So utterly distracted by everything, frustrated by the idea of a monster he can't see or feel or hurt, Merlin is almost relieved when Gwen skids into Arthur's half-tidied room and drags him out, telling him that Gaius has collapsed and can't be woken.

He stumbles to the court physician's rooms, feeling sick; it's not a good sign when Donna wraps an arm around Gwen's shoulders and herds her out despite her protests. Gaius is slumped across his bed, eyes glassy, and the Doctor is watching the way his limbs are trembling. There's an odd metallic device of some kind on Gaius' workbench, lighting in a constant flicker like a guttering candle.

"Tell me why you're here," the Doctor says to Gaius, his face ashen. He glances at Merlin only for long enough to acknowledge his presence. "I know your host has gone, but you don't have to keep doing this. You can come with me. I'll find you a new home. Just stop hurting these people."

"There is no new home." It's Gaius, but it's not - the voice is rusty, unpracticed, like someone pretending to be Gaius who's never spoken before. "You would only have me dead."

"I won't - I swear to you," the Doctor says, urgently. "Tell me how this happened."

"My host...she was discovered, when I was feeding. They...threw her away. She died in this planet's atmosphere. I took their captain, and I...I drove their ship until it burnt, as they had burnt her."

He says it with a grim satisfaction, as if he's avenged the death of a loved one; Merlin frowns. The Doctor described symbiosis coldly, as a practical arrangement, but it seems to be so much more.

The Doctor touches Gaius' shoulder. "I'm sorry."

"I fell to this world. This rich, living world. What use would I have for another host?"

Merlin understands with a curious clarity: this thing is burning, too, in its grief and its loss. It won't want another host, it will never want to risk that intimacy again, even at the cost of its own life. It loved her, and she died, and no amount of vengeance wreaked will be enough.

"Stop," the Doctor whispers.

"I will." Gaius shakes and curls, like a fist clenching, and the moan of pain that escapes his mouth is undeniably his own. "When this one is dead, it will all be over. I am losing my strength. You feel it. This is not enough."

"Don't do this. I can help you."

They're creatures of pure energy. The only thing that can hurt them is what they consume.

Merlin knows what he has to do.

"Take me," he says, moving forward. "You've noticed me, haven't you? You can't have not noticed me, it's like something tries to kill me every other week. Leave Gaius. Take as much as you need, but take it from me. I'm begging you."

"Merlin, don't - "

Merlin grabs Gaius' hand, feels it cold and clammy in his grasp.

It comes in a cold flood, a solid wave of pain: it swallows him, tearing through his veins and ripping out everything of value. He can feel his magic still, at a peculiar distance, inside it inside him. There's a dull roar in his ears, behind his eyes. He can feel his skin cooling.

Merlin thinks, suddenly, of Arthur sparring with Sir Pellinore. Pellinore was disarmed, but he clobbered Arthur with his shield and took the Prince's sword. In his hands it was a clumsy, useless weapon, but in Arthur's it was perfect. Because it was his, always his.

"I'm sorry," Merlin says, or maybe he only thinks it. Then he lets his magic burn.

It howls, throwing Merlin's body to the floor like a doll; it dies, obliterated from the inside out. But the wildfire isn't stopping, and Merlin feels the ragged edges where his magic becomes his own essence, his own soul, and that's burning too and he knows he's done too much, and he's really going to die this time and this can't be stopped or healed because he's done it to himself.

He hears the Doctor calling to him, then Gaius speaking in panic and confusion, but it's all a very long way away. Everything feels hot, too hot. He feels his own sweat on his skin, and his nose starting to bleed.

"Stop. That's enough. Let it go."

There's cool, suddenly, in a broad press against his back and a bar around his waist.

"Stop it. You know you can do it. Make it stop."

A breath in his ear, a hand clutched over his chest where his heart is on fire.

"Follow an order for once in your life, you useless dolt. Stop. Stop..."

Merlin thinks of cold water, and darkness.

He lets the voice loop around him and hold him steady, and there's a second voice:

"He's breathing. I think he'll pull through. He's strong, you both are."

"Idiot. Complete, utter, inexplicable..."

"Take care of him, Arthur."

"What?"

"I know, I know, I don't even have to say it."

"....No. You don't."

Then Merlin knows it's safe to let go, and there's nothing.

****

"It's dead," the Doctor says quietly, leading Donna back to the TARDIS. "He let it gorge itself, then detonated his own power. Impressive, really."

Donna, who spent the whole time preventing Gwen and then Morgana from seeing more than they had to, is a touch irritated to have missed everything. She nods.

"I don't think it ever would have left," the Doctor says. "It just wanted to keep killing."

"It's for the best, then," Donna says firmly.

"I suppose. I just hoped..."

"It's for the best. Come on."

They get another twenty paces or so.

"Your door-guarding skills weren't up to much," he says, although it comes out sounding grateful.

"My door-guarding skills are selective, thanks. So what happens to Merlin, anyway?"

The Doctor smiles. "Oh, he becomes the greatest sorcerer in human history. I thought you knew that bit, Little Miss Fick-shun."

"Oi, leave it out." Donna tucks a hand in her pocket and rummages. "Still, it seems...weird."

"What does?"

"That he's the greatest whatever in whenever but you've never met him before."

"Hm? Never said that. Merlin and me, we go ages back. Bump into him all the time. He owes me a drink, as it happens. Just...not yet, not for him." The Doctor watches Donna unlock the TARDIS door and push. "This was his first time meeting me, but it's not my first time meeting him."

"So why'd you do the whole thing like you'd met him for the first...oh. I get it. Spoilers?"

"Spoilers."

****

Merlin hears a little: Gaius telling someone not to stress him, that he's very weak still, and then he hears that someone scoffing derisively. His eyelids feel weak and heavy; his whole body feels weak, and hot with fever. His arms and legs feel disconnected.

A door closes. A damp, cold cloth swipes his brow, not rough and businesslike the way Gaius does it, nor gentle-yet-ineffectual like Gwen. Drops of water trail down his face, over his cheekbones and across his lips. He lets his mouth open. He's parched.

The rim of a wooden cup presses to his lower lip, and Arthur says: "Drink. Slowly."

The water is sweet and cool. Merlin drinks until some goes down the wrong way, and he splutters, and Arthur tuts.

"I've told my father that the beast came up against a sorcerer who sacrificed his own life to destroy it. Don't give him any reason to think I've lied."

Merlin opens his eyes, focusing slowly; Arthur looks pensive, his face tight and drawn.

"I knew it would come for you. I just didn't realise you would be stupid enough to actively throw yourself at it."

Merlin stares. "You knew. You knew all this time."

"Well, yes. You've done a rather terrible job at keeping it secret."

"Why?" Merlin asks, arrested.

"Well, I didn't quite know until after you had to go and save my life in front of everyone, when I was training with you for the tournament. That was when I realised you hadn't the reflexes to pull me out of the way of anything, not without help, and from there it was just a matter of paying attention - "

"No...I mean, okay, but that's not what I....I meant, why? Why not tell your father?"

"Would you rather I did?"

"No-!" Merlin sits up, feeling immensely uncoordinated still. "I just...I want to understand. That's all."

"Can't believe you have to ask," Arthur says. "Do you think I'd be able to do this without you?"

He doesn't specify what 'this' is, living maybe, but it doesn't matter, because all of a sudden he's close to Merlin and then leaning down and kissing him. Merlin feels it again, the dark cool magic in his bones, and he reaches out to touch Arthur's face, and then nothing else matters.