We're at an oasis in the middle of a desert. The trees are made out of bones, flaps of skin instead of leaves, and the well at the centre is filled with a dark, sulphurous liquid. The liquid's alive and can suck in and kill passers-by, but it only has a reach of two or three metres, so as long as we don't stray too close to it, we're safe.

The oasis was designed by a demon master a long time ago, based on something he'd seen on Earth. As much as demons hate humans and our world, they envy our forms and shapes. That's why many of them base their bodies on animals from our planet. They lack our imagination or the skills of Mother Nature.

We've been here for a week, although it's hard to judge the passage of time. There's one sun and moon above the oasis, like on Earth, but they never move. The sun shines for hours on end, holding its position in the sky, then abruptly dims to be replaced by the light of a three-quarters full moon.

I haven't had to eat or drink since I came, and I've only slept twice, a couple of hours each time. The magic in the air is far thicker than it was on my world sixteen hundred years ago. I could perform amazing feats here, turn a mountain upside-down if I wanted. The trouble is, if I can do that much, so can the demons.

We haven't seen any of the Demonata yet. This is an abandoned region. Its master moved on or died, leaving only the skeletal trees and cannibalistic well. Individual demons wander through occasionally-some are picked off by the well-but incursions are rare. Beranabus has used it as a bolthole on several occasions.

Sharmila is still recovering, but we haven't been able to restore her lower legs. Magic works differently in each person. Kernel was able to replace his eyes when he lost them, but Sharmila can't grow new legs. You never know for sure what you can or can't do with your power until you test it.

Beranabus and I have used some of the bones and fleshy leaves from the trees to create artificial legs. We've attached them to Sharmila's thighs and she's spent the last couple of days adjusting, using magic to operate the limbs and keep her balance. She moves clumsily when she walks, and with great discomfort, but at least she's mobile. I don't know what will happen when she returns to the human world-the legs we've created won't work in a place without magic-but for now she's coping.

Dervish looks healthier too. I've taught him ways to direct magic into his heart, to strengthen and protect it. He should be fine as long as he stays here, but if he returns home the situation will rapidly change. His heart won't hold up long over there.

Dervish wove the material of his hospital dressing-gown in with scraps of flesh from the trees to create a costume. He's also given himself a full head of silver hair and stuck it up in six long, purple-tipped spikes. I was startled when I first saw it.

"I had spikes like this the last time I fought alongside Beranabus," he explained, blushing slightly. "I walked away alive then, so maybe they were good luck. We'll need all the luck we can muster when we fight again."

There's no doubt we'll have to fight, either the Shadow or its army. The first battles have already been waged. Before Meera and Shark tracked them down, Beranabus, Kernel and Grubbs were flitting from realm to realm, hunting demons and challenging them, trying to find out more about the mysterious Shadow.

We saw the Shadow the night Bill-E was killed. A huge, nebulous cloud of a monster, darker than any night, almost as black as the cave when I was sealed up there. Immensely powerful even by demonic standards.

Lord Loss said the creature would destroy humanity. The maudlin demon master craves human misery, feeding on it like a cat slurping milk. In my time he slyly helped me close a tunnel, to stop a demon invasion. He needs humans the way a fish needs water.

But he's afraid of the Shadow. He doesn't believe mankind can defeat this new threat. He sided with the creature, served as if he was an ordinary demon, not a powerful master. He did the Shadow's bidding, even though that might mean the end of the human suffering he cherishes.

Lord Loss's fear of the Shadow fills Beranabus with unease. He believes the war between humanity and the Demonata can't last forever. In the distant past, the powerful Old Creatures ruled the Earth and demons couldn't cross. By my time their power had waned. That led to the current war between humans and demons. Beranabus thinks we must find a way to block their passage between universes or they'll wipe us out completely.

The Kah-Gash has been Beranabus's only real hope. According to the ancient legends, the weapon can destroy a universe-ours or the Demonata's. He'd love to do that. It doesn't bother him that he'd be eradicating an entire life form. He sees this as a blood-drenched fight to the finish. The universes are colliding and only the victors will survive.

Beranabus has the Kah-Gash now-in the shape of Grubbs, Kernel and me-but he doesn't trust it. The weapon has a will of its own. It worked through us when the world was last threatened by a demon breakthrough but it's been silent since. We don't know what its plans or desires are.

Beranabus hoped to experiment, unlock the Kah-Gash's secrets, find out how to direct its great power. But so far he hasn't learnt anything new.

Unwilling to unleash the Kah-Gash, Beranabus instead hunted for the shadowy monster we'd glimpsed in the cave. Having no name for it, he dubbed it the Shadow. The more he chased it, the more apt that name became.

Beranabus has interrogated many demons who know about the Shadow, but not one knows its real name. It's rumoured to be more powerful than any other demon. They say it's been working in secrecy for hundreds of years. That it recently made itself known to a number of demon masters, recruiting them to help it achieve its ultimate aim-the removal of the human stain.

That's how demons see us, as a stain on the universes. They were here long before us and consider themselves superior. They hated the Old Creatures but respected them. They have nothing but contempt for our weak, mortal kind.

The Shadow has promised to kill every human and make the Demonata more powerful than ever. A few demons told Beranabus that it had even promised a return to the original state of the universes and the elimination of death, but we're not sure what that means. The demons weren't sure either.

Beranabus hasn't dared go after any of the masters. They're too powerful. He thinks the creature has made its base in Lord Loss's realm, but he dares not set foot there. And Kernel-who can usually find anything in either universe-isn't able to search for the beast since he doesn't know the thing's name and didn't see it in the cave, being blind at the time. Beranabus has tried to magically recreate a picture of the Shadow, but it always comes up blurred and indistinct.

We spent the first couple of days here arguing about what to do next. While I worked tirelessly on Sharmila's legs-and helped her adjust to the shock when she regained consciousness-Dervish pressed Beranabus to focus on the werewolf and demon attacks.

"You've been chasing this Shadow for months without result," he argued. "This is something concrete, a puzzle we can solve. Better to direct our energies at a problem we can crack than waste them on an enigma."

"But all else is irrelevant," Beranabus bellowed. "The Shadow is the greatest threat humanity has ever faced. We have to pursue it relentlessly, down as many blind alleys as it takes, until we find a demon who knows its name, where it comes from, how powerful it is. The knowledge is out there. We just have to find it. But we can't do that if we squander our time on a bunch of hairy Grady miscreants!"

Dervish countered by insisting the attacks were linked to the Shadow. We know Lord Loss is working for him, and that the revived Juni Swan works for Lord Loss.

"Maybe Lord Loss and Juni just want to kill us before the world is ruined," he said. "But they might be planning to use the werewolves to target the Disciples, kill as many as they can and clear the way for crossings."

Kernel supported Dervish. "We can't go after Lord Loss directly-he's too powerful," he said. "But we can target Juni. Lord Loss didn't show himself at the hospital but Juni was acting on his behalf. She might have been part of the group in Carcery Vale too. If more assaults on the Disciples are planned, she'll possibly act as the go-between again, conveying Lord Loss's orders to their allies. If we can trap her, we can find out what she knows about the Shadow."

Beranabus was swayed by that and told Kernel to devote himself to tracking her movements. I think he's keen to get his hands on Juni for personal reasons. She betrayed him. But it's not just revenge he's interested in. He also wants to know how she came back.

We don't understand how my soul remained trapped in the cave when I died, or how I returned to life. That's never happened before. Ghosts exist, but they're mere after-images of people. We don't know where a person's soul goes when they die, if there's a heavenly realm, if they get reborn or if they simply cease to exist. But they always move on. Never, in all of history, has a person's soul survived death and returned to life. Until me. And now Juni.

Beranabus believes I survived because I'm part of the Kah-Gash. The mystical weapon turned back time, so it could feasibly cheat death too. But Juni isn't a piece of the Kah-Gash. She shouldn't have been able to survive the destruction of her body. Her return troubles Beranabus deeply. He suspects it's linked to the rise of the Shadow. If the new demon leader has the power to restore life, maybe it shares other powers in common with the Kah-Gash. Beranabus wants-needs-to know.

So Kernel has been focusing on Juni and Lord Loss for the last few days. He's developed in many ways since the three of us worked in league as the Kah-Gash. He can do more than open windows now. He can search for several people at the same time, and track their movements-he knows when they switch from one realm or universe to the other.

Juni is currently in Lord Loss's kingdom, with her master. But as soon as she moves, Kernel will know and we'll blaze into action.

I've spent a lot of time with Beranabus. He's changed so much over the centuries, made himself hard and uncaring, believing he had to be like a demon in order to fight the Demonata. It helped that he is half-demon. There's a monster within him, always active, struggling to rise to the surface. Beranabus has to fight it constantly to maintain control, but through those battles he's learnt more about demons and their ways than he ever could have otherwise.

One of his greatest fears is that he'll go insane and the demon within him will take over. It would be the ultimate irony-the man who spent all his life battling to save humans from the Demonata turns into one of them and goes on a massive killing spree.

Beranabus can discuss such fears with me because I already know about them. I absorbed his secrets along with his memories, so he can't hide them from me. I know almost as much about the ancient magician as he does.

"Sometimes I wonder if my life's been worthwhile," he muttered last night when we were apart from the others. "I've gone without pleasure or company for most of my years. If we lose and the Demonata kill us all, there won't have been a point. Maybe I should have settled, married, had children, lived a normal life. It might not have made any difference in the end."

I tried to make him see that millions of people owe their lives to him, that the Demonata would have taken over our world many centuries before this if not for his stubborn resistance. But he's fallen into a dark state of mind. I think partly it's because of my return. I've made him aware of all that he's missed out on. If he'd allowed himself to be more human, he'd have had friends and family, and perhaps been much happier.

I'm sitting beneath the shade of a bony tree, trying to think of a way to ease Beranabus's troubled mind. Someone coughs close by, disturbing me. I open my eyes and find Dervish standing there. "Mind if I sit down?"

I nudge over. When he's sitting, he smiles awkwardly. We haven't said much to each other since he recovered. I think he's embarrassed-we'd had that big conversation prior to the attacks, but never had a chance to wrap it up.

"How are you getting on?" he asks.

"Not too bad."

"It's pretty boring here, huh?"

I shrug. "I'd rather this to the excitement of fighting demons."

He strokes one of his newly grown spikes. "What do you think of the hair?"

"Some of the warriors in my time styled their hair like that," I tell him.

"Yeah?" He looks proud.

"But they were all a lot younger than you."

He pulls a face. "I started going bald early, so I had no choice other than adapt. But I never liked looking like the crown of an egg."

"Baldness suits old men."

"I'm not..." he starts to protest, then sighs. "No, you're right, I am old. It happened while I wasn't looking. Old, bald, dodgy heart, ignorant."

"Ignorant?" I echo.

"The way I treated you," he says softly. "I was an ignorant old man. If Billy or Grubbs had seen me acting that way, they'd have kicked me hard and told me to stop being an idiot."

"You were upset," I excuse him. "People do strange things when they lose a loved one."

"I should have known better," he grunts. "I would have been more sensitive a few years ago, but you don't see things so clearly when you let yourself become a grumpy old fogey. I used to criticise Ma and Pa Spleen-Billy's grandparents-for being cranky and small-minded. But I was turning into a carbon copy of them." He shudders.

"Bill-E loved his grandparents regardless of their flaws," I say. "He would have gone on loving and forgiving you too, no matter what."

"How about you?" Dervish asks.

I frown uncomfortably. I should say something diplomatic, but I was reared to speak my mind. "I don't love you. I hardly even know you."

"I didn't mean that," Dervish says quickly. "I meant, can you forgive me? Can we be friends? Or will I always be the ogre who made you tell him stories about a dead boy for months on end?"

"You'll always be an ogre," I say seriously, then laugh at his expression. "I'm joking. Of course we can be friends."

"We can start over?" he says eagerly. "Get to know each other properly?"

I nod and he sticks out a hand to shake on the deal.

"You know about my gift?" I say hesitantly.

"Yes. But I don't care. You don't hold things back from friends."

I smile, then shyly shake his cool, wrinkled, welcome hand.

Kernel is off by himself, doggedly monitoring Juni's position. The rest of us are duelling, practising our skills, learning. It's difficult to define your magical limits. Magic is a mysterious, ever-shifting force. You can test yourself in certain ways on Earth, but you never know how far you can stretch until circumstances compel you to improvise.

Sharmila told me that when Kernel first came to this universe, Beranabus threw him at a flesh-eating tree to establish his magical potential. When his life came under threat. Kernel reacted and he fought free. If he'd been of lesser potential, he'd have perished. That's a cruel way to test a person, but there's no easy alternative. Magic is part of a harsh universe. Those who wish to channel its power must accept that.

Beranabus sends twin balls of fire shooting at Dervish and me. I turn the ball aimed at me into an icy mist, but Dervish isn't as swift. He disperses the flames, but not before they singe his beard and redden his cheeks and lips.

"You're slow," Beranabus grunts while Dervish repairs the damage.

"So are you!" Sharmila shouts, hitting Beranabus with a burst of energy from behind. He shoots forward, yelling with surprise, and smashes into a tree, sending bones flying in all directions.

"That hurt," Beranabus complains, staggering to his feet and rubbing the small of his back. He bends to pick some splinters out of his bare feet. We've all got rid of our shoes-they hinder the flow of magic.

"Be thankful I was not aiming to kill," Sharmila says coolly. "We are all slower and weaker than before. It is the penalty of old age. No one can avoid it."

"I've done better than most for a millennium and a half," Beranabus growls.

"But time catches up with us all eventually, even you."

Beranabus twists slowly left, then right, working the pain out of his back. "I suppose you're right," he grumbles. "I've known for a long time I'm not as quick or powerful as I once was."

He waves a hand at Sharmila and her artificial legs snap apart. She collapses with a yelp of shock and pain.

"But there's life in the old dog yet," Beranabus shouts triumphantly, before guiltily hurrying to Sharmila's side to fix the damage.

Kernel has kept himself distant, sitting in the open with his legs crossed, tinkering with the lights that only he can see, keeping tabs on Juni. Beranabus told me his bald assistant finds it hard to focus these days. Since he got his new eyes he's been seeing patches of light which were invisible to him before. He can't control the new patches and they distract him. He's been trying to ignore them, but I often spot him scowling and cursing, waving an irritated hand at the air around him.

In the middle of another dry, lifeless afternoon, as the others are resting while I leap from tree to tree testing my powers of flight, Kernel uncrosses his legs and stands.

"She's moving," he says.

We're by his side in seconds. His bright blue eyes are alive with flickering spots of light. He looks nervous.

"Where did she go?" Beranabus asks.

"Earth."

"And Lord Loss?"

"He stayed in his own realm."

"Can you tell where exactly she is?" Dervish asks.

"No." He frowns. "I should be able to, but I can't place it."

"Is she close to Grubbs?" Dervish presses.

Kernel concentrates, then shakes his head.

"Well?" Sharmila asks Beranabus.

"Kernel and I will investigate. The rest of you stay here."

"Nuts to that," Dervish huffs.

"Don't forget about your heart," Beranabus says. "Or Sharmila's legs. You're a pair of wrecks on that world. Let us check the situation and report back. We won't engage her if we can avoid it."

"What about me?" I ask. "I can survive there."

"Aye, but I'm asking you to wait. Please. Until we know more about what we're walking into." I don't like it, but I know Beranabus worries about me. Better to go along with his wishes, so he can operate free of any distractions.

Kernel opens a window within minutes. It's a white panel of light. I think I can smell the real world through it, but that's just my nose playing tricks. Without saying anything, Kernel steps through, Beranabus half a step behind him.

"We'll give them five minutes," Dervish rumbles. "If they're not back by then, we-" Beranabus sticks his head through the window, catching us by surprise. "It's an area of magic," he says. "Sharmila and Dervish will be fine there. Come on."

He disappears again. We glance at each other uneasily, then file through one by one, back to the human universe, in search of the semi-human Juni Swan and a host of shadowy answers.