The citadel is as tall as a thirty-storey building and featureless as a brick. Nat walks around it once, looking for windows and gates. Finding none, she tries imagining herself inside and then "stepping into" the mental image, which is how things usually work here. It doesn't work. Abandoning subtlety, she makes one hand gluey and uses it to pull a black stone as big as a refrigerator right out of the wall. She flings it away over her shoulder. That leaves a large rectangular slot, with just enough room for her to clamber into the dark place inside.

The wall is a metre and a half thick, with nothing behind it but a half-metre gap and an identical wall. Natalie shuffles sideways along the narrow alley until she reaches the corner, where both walls turn ninety degrees. She looks up, and can't see a lot, but there's definitely clear space above her head. She concludes that what she's looking at is a second, slightly smaller citadel inside the first one.

Everything goes dark. The outer wall has repaired itself. Something throbs in Natalie's brain, the overspill from the change to the universe. Somebody did that. Natalie creates a light for herself, red so as not to ruin her night vision.

The fuzzing, shuffling background noise, which, although still soft enough to tune out, has been patiently increasing in volume since they both arrived here, ceases.

That's right, Natalie remembers. If you stay here long enough, it becomes a nightmare.

She shuffles around the corner and along the alley a little further, just on the off-chance that there's an entrance on this wall. All she finds is flat walls and an ominous silent darkness stretching away an unknown distance. " Sedo sedo sedo sedo, " she says, catching the paint as it falls out of her mouth, gathering it into a squishy ball, a water balloon without the balloon. " Eset eset eset, zui zui zui. "

Then she turns around and hurls the paint blob at the eyes of the genuinely horrific, wasted individual which, by all nightmare logic, had to be behind her. She throws the paint in its eyes, not at its head, which is an important distinction. Where a normal human would have a face, it has a huge vertical mouth, opening to show two rows of fangs. Where one would expect shoulders and arms, it has rows of long feely fingers, as if its whole torso was a palm for holding things where they can't escape. Most of them are perfectly normal human fingers, merely anatomically misplaced. Two of them, Natalie discovers as she turns, have lengthened and reached out, bracketing her neck and beginning to close. There are no slavering sound effects; the thing is as silent as a spider.

And its eyes are in its neck, above the collarbone. The eyes are human enough that the thing has something approximating a facial expression. The expression is near-madness.

Nat's paintbomb obliterates its vision. As it stumbles backwards, its long fingers brush across the back of Nat's hair. She grabs a finger in each fist and squeezes, snapping them like twigs. Then she places a hand on another stone in the outer wall and pulls it into the alley as far as it will go, pulling away just in time so that the stone can crush the thing flat against the inner wall with its momentum. She grabs the stone and slams it against the crushed horror a few more times, just in case. Under this light, she couldn't guess what its skin colour is, but its blood is definitely black. And runny, like wine. There's enough of it that it must be dead.

High energy magic. High energy dreams. Natalie wonders what the concrete, real-world dangers are if one of those things eats her. Laura would know.

Natalie climbs onto the stone she just produced and looks as far as she can in every direction, casting her light up and down the alleyway and even above her. There are more things coming, making no noise but a busy shuffling. It is hard to see them but they are all permutations of human bodies, with features and spindly extremities multiplied and redistributed. They are dirty, and many of them appear to be able to climb, and they are converging on her. If it was actually happening, it would be the most frightening thing to ever happen to her.

She thumps the inner citadel wall. Its stones won't move. "Let me in! I'm real." Something picks her up around the waist and begins dragging her upwards. She can't see this one clearly, but the sensation is grotesque. She flings more paint at it, then braces herself against the inner wall to kick a hole in the outer one. The dislodged stone lands somewhere outside. A little pale light floods in, but is immediately blotted out by more malformed people flooding in from outside. They wrap her in their fingers and hold her tight. They start chewing at her toes, clothes and hair. And as one final problem, off to her far right down the alley, a brilliant green-white light flares into operation and starts moving towards all of them.

Natalie sees what the light means. It's an abstract. It's not part of the nightmare. It's an agent of something else entirely. It is a sweep. While she watches (out of the corner of her eye, because she can't turn her head), she sees "people" evaporating as it catches them. The green hue represents disinfectant. The white intensity represents heat sterilisation.

Natalie wonders if it'll discriminate, or just fry her too. She wonders if it'll wake her up or just kill her. Maybe it's a metaphor for the magma: it'll wake her up and then kill her. She screws up her eyes as the light sweeps over her and she wonders: Tanako's world is a nightmare. I'm not scared. So whose nightmare is it?

When she looks again, she is inside the second citadel. And inside the third, the fourth, the fifth and the sixth citadels. It is a tiny windowless studio apartment, lit dully by an uplight in the corner. There are bookshelves and a table and a bed. There are no doors. There's a man curled up in the bed, facing the wall. His hair is all that's visible. It's slightly blond and slightly spiked.

"I'm Natalie Ferno," says Natalie. "Did you build this place? How long have you been here?"

He rolls over. "You have to believe me."

"I believe you."

"He's not me."

"I believe you. I'm real. I can help you."

Benj blinks. "Can you get me out of here?"

"Yes."

*

Before she gets within a kilometre of Krallafjöll, the ground beneath Laura's feet has begun to melt. Soon she is walking over alternating sections of lava and black pan-hot rock. Smirking, she cranks up imaginary protective spells using her rings and staff. She imagines invisible iron armour and big heat sink panels like wings, all weightless. She skips straight through to her planned Mark Two defensive shield, which clings to her clothes and skin at a one-millimetre thickness, but lets cool air through so she can breathe and perspire. It's much easier to build this stuff in her head. "Do What I Mean."

While walking, she keeps catching sight of something out of the corner of her eye, but every time she turns to look at it it disappears. She has the sensation that someone is walking beside her. Not stalking. Just... accompanying.

She arrives at the nearest end of the ridge. It looks the part now: active, volcanic. Half of one side of it is missing, with lava rolling out of the wound and running downhill into a substantial pool at the foot of the ridge. The pool has swollen, and excess lava has begun to flow out of it towards Blönflói.

The whole scene is almost stopped in time, just barely crawling forward if she watches carefully, like the minute hand on a clock. At the top of the hill, Benj is still standing. It seems he's not the light source that she saw from her castle. The molybdenum steel ring which he's holding, which he must have recovered, is glowing as if white-hot. The mana radiating off it is so concentrated that it stings Laura's eyes and causes jangling perturbations in her shields.

Laura quickly scales the ridge, coming up behind Benj, who is gazing into the lake of lava gathering below them. Benj turns around and smiles broadly. And it's Benj. It's really him. The face, the body language. There's nothing about him that suggests he's anybody else.

"Neat trick. Who are you?" Laura demands.

"I'm Benjamin Clarke. We have met, you know."

"Guess again."

"I'm an animate stray thought," he says. "There was an accident. You remember. You, me, Jeremy Willan--"

"Not you. It wasn't you. It was Benjamin Clarke. Who are you?"

"...You, Benj Clarke and Jeremy Willan were locked into a shared trance, this world that Kazuya Tanako discovered. And you know what happens here. There are physical risks to staying here for too long. There are things which crawl out of cracks in the glasswork, and it never ends prettily. They got to Clarke first. So he ran away and tried not to be found. And he succeeded. You and Willan and Dan Czarnecki couldn't find him. You used dream logic to cook up a cheap facsimile of him. And you brought that home instead.

"Clarke's been here this whole time. Holed up in his castle, fighting nightmares and unable to wake up. But occasionally getting messages through." He imitates mockingly: "'This isn't me!'"

"You haven't answered my question," Laura says angrily. "What does that make you? A brain-damaged version of Benj? Jeremy built your shell, I animated it. So where are your motivations coming from? Me? Benj? Who is ra ?" With the last word she spits lime green goop on the ground between them.

The "Benj" facsimile shrugs. "Everything I just told you was information you already had. I've got nothing else for you. You're not actually talking to anybody right now, Ferno. You're asleep."

"No. I think you're really here. That's why we've overlapped into Benj's dream. I think you've been dragged in here with us. What are you trying to do?"

"Look." Not-Benj points down at the lava pool. She looks. There are two figures lying in the pool, face-down. The figures are wearing familiar cold-weather gear. Laura recognises herself and her sister. Of course: give or take the heat shielding question, human beings float in lava. Bowling balls float in lava. It's molten rock; it has about the same density as solid rock.

Laura wonders how closely this mental image resembles reality. If she and Natalie really are unconscious on the lava's surface, they can just get up and start running - inside their shield - to get to safety. No mucking about with the highly complex, untested tunnel boring spell. That would remove one variable, leaving - by their count - three. Provided that what's she's dreaming is also real.

But as she watches, the force field protecting the figures of her and Natalie pops, and they plop into the lava. They catch fire, melt and explode simultaneously. The organic matter produces steam, which makes the lava gulp and spurt as it swallows the bodies up. Laura stares, transfixed. The reaction seems to take a long time.

"You just died," says not-Benj.

Laura grits her teeth. It is, of course, impossible that she could be dead. Impossible and deeply worrying.

"You said this was about freedom," she says.

The imposter twirls his glowing ring around an index finger. If Laura listens carefully, she can hear its deep, quiet synthesised voice still speaking its recursive spell. He says, "Do you know what's the most mana that's ever been gathered together in one location? The mana energy density record?"

"Why, set on breaking it?"

"Breaking something. Do you know what happens when you put too much mu and zeta in the same place?"

Laura takes a cautionary step backwards. Looking back, she sees that Nat has caught back up with her, wearing a shield to protect her from the heat of the lava she just crossed. In tow, sharing the shield, is Benj. The real Benj. And beyond them, in the distance, she sees that Tanako's glass world and the two castles are gone, replaced with dark mountains. That'll work, she realises. We'll bring the real Benj home with us when we wake up. That might actually work.

As far as she understands - and she likes to think that she understands a great deal - nothing special happens at all when you put too much mu and zeta in the same place. At least, in theory. But nobody's done it. How could they? There's nowhere to get that much mana from.

Laura aims one arm full of amulets at the imposter and holds her staff out backwards with her other hand. "Last chance. Tell me what you're trying to achieve. This is a dream. I can do anything here."

"So can I."

"But I can think faster than you."

The molybdenum ring is so bright now that not-Benj's fist is glowing bright red, with dark spots where his bones are. This reminds Laura of something. She can't remember what. In answer to her question, not-Benj produces a dark grey sphere from one of his jacket pockets. It is the size of a baseball, matte and smooth, and seems to be very heavy. It's so heavy that Laura thinks she would have noticed it weighing in his pocket before now. He must have just created it. It's so heavy, as if it was made of some sort of--

dense metal--

She triggers the last three spells reflexively. By the time the words "implosion assembly" and "subcritical plutonium core" have formed in her mind, the whole thing is over.

" Dulaku ragígakal! "

The first spell is a green laser powerful enough that its beam is visible in clear air. It slices through not-Benj's magic ring with a noise like two bolts being cut. The chain reaction spell cuts out mid-syllable. Half of the ring falls; not-Benj is left holding a C-shaped chunk of metal. Enough stored mana to flip Reykjavik like a pancake crackles out of the four exposed ends, and that would be the end of all of them, plutonium or not, but Laura reaches out and catches the magic with her staff, funnelling it into spell two.

Two is the heavyweight, the figurative tank, the one she could never have built without Natalie's help. With an audible whump, the local environment out to a range of about a kilometre and a half drops in temperature, hard. The entire fissure is instantly frozen solid, including all the visible lava, which sets into rock. That's the village saved.

Laura, Benj and Natalie all have shielding against the thermal attack. Not-Benj has shields, but not the right ones. He turns blue.

Now Laura just has to deal with the newly-liberated thermal energy rushing back the other way. With another flick of her staff she pipes it all into spell three, her old heat lance. Not-Benj might as well be standing at point blank range in front of the Sun. He simply ceases to exist; he, and his half of the ring, and his plutonium baseball. All three spells are over in a split second.

Someone faint is beside Laura. Even looking right at him, all she can see is a slight crystalline shimmer at the edges, as if he was a shell of cobweb-thin glass. He puts a hand on top of her outstretched arm and gently but firmly pushes it down, safely aiming all of her thaumic weaponry at the ground.

*

They've stumbled about a quarter of the way back to the village when they meet the Jeep coming in the opposite direction. Tómas Einarsson is driving it, with Steve Aldridge in the passenger seat. It's Aldridge who jumps out as soon as the vehicle stops and rushes over. "Is anybody injured? Anybody get burned, anybody inhale any ash, any SO 2 ?"

"It's fine, we're fine," Laura explains. "We ran for it and we're okay. I think there was a minor eruption and it's stopped now."

"Are you kidding me? You think you're qualified to make an assessment of that? Get in the car. All of you! Tómas, let's go."

Tómas has been studying the ridge through a monocular oracle. Without a word, he slots it back into his inner pocket and pulls the Jeep around to head back towards Blönflói. The road is bumpy and the ride is fast. They'll be back to the village in a matter of minutes.

"I'm not angry," Aldridge explains as they drive back. He was clearly frantic on the way up, but is now visibly cooling off. "You told people where you were going. You did nothing wrong."

"Is the village evacuating?" Benj asks.

"Yes," says Tómas. "The village is so close to the fissure that we do not take chances. The evacuation plan is very paranoid. Although, the eruption has stopped. So they will probably reverse the order in a few minutes."

"Did you radio in that we've got them?" Aldridge asks him.

"Yes. Also, this has never happened before. Krallafjöll has been inactive for a very long time."

They drive in silence for a minute. Aldridge turns around in his seat. "What happened up there?"

"We don't know," says Natalie, immediately.

"Were you doing magic?"

"We were meditating. I was meditating. The others were looking at the view. We don't know what happened."

Tómas is grinning and shaking his head at Aldridge. "It wasn't them. Everybody tries to set off Krallafjöll. It was bad luck."

Instead of stopping at the hostel in the village, Tómas turns onto Route 1 and follows several other vehicles up to the evacuation point, which is entirely on the other side of a hill from Krallafjöll and therefore - within reason - out of harm's way. There are about a hundred people there already, including all the other students and UK and Icelandic staff. They get out and gather in a sheep pasture where they can see the ridge. It's eventually revealed that since this is the first time the village's evacuation plan has been used in anger, it's being seen through to completion as a semi-live drill. There are volcanologists running around with walkie-talkies. The fissure is totally dark and silent. The air is bitterly cold but the sky is clear and starry. All they'd need is a bonfire and it'd be an event worth attending.

Natalie pulls Laura aside. "I need you to do something for me."

"Okay?"

"I need you to stop killing people."

Laura chokes. "Ah, what?"

"Benj!" Natalie shouts. Benj is nearby. He turns around. "Someone wrote QUINIO. Was it you?"

"No," he says.

"Then who wrote it?"

"I don't know. That guy."

"You killed that person," Natalie says to Laura. "You could have vented the energy upwards, away from all of us. You vaporised him and you didn't need to."

"He was threatening lives!"

"You had neutralised that threat. That makes it murder." Natalie's tone of voice isn't even accusatory. It's completely flat.

"I don't understand what you're saying. It was a dream."

"A dream that real counts as real. Any other dream, do whatever you like. But a dream with real people in it, you and me and Benj, and that much power behind it, you need to take more care. If you'd directed that thing at me, I wouldn't be here. It counts."

"It was figurative. It was metaphorical. None of it happened."

"None of it? The ridge exploded. You froze it solid. With a spell. And there're going to be questions about that, for one thing, but you did it while asleep."

"I wasn't asleep, I was--"

"So when did you wake up?"

Laura blinks.

Benj says, "I don't remember waking up."

"Me neither," says Natalie. "Laura?"

Laura steps through her memories one at a time. She remembers walking home. She walked all the way back from her memory palace to Tanako's world to the fissure to reality. It's a continuous record. "...We're awake right now, right?"

"That's the null hypothesis," says Natalie. "Until you can prove otherwise, always assume you're in reality. Now, repeat after me: 'We don't know what happened.'"