Light flares dimly in his left hand. Slowly, he lets it grow and expand, filling his palm and then rising to hang in the hot air above us. It lights up the entire cave, revealing a site of beautiful wonders and a wretched terror.

The wonders—V-shaped, glistening formations of a substance not quite rock. Some reach up from the floor, others hang from the ceiling. All sorts of sizes. Water drips from the tips of some of the overhanging shapes, to splash over the floor of the cave or one of the uprising Vs. In some places it’s as if the shapes are reaching for each other, growing towards one another.

There are other formations stretched between the floor and ceiling, some huge, others tiny bulges. And an underground waterfall to our right, the water appearing as if by magic from high up the wall, vanishing through a crack in the rocks underneath, flowing on to who knows where.

This is what I imagine the Otherworld or Tir na n’Og to be like. It doesn’t feel as if it belongs to our world. It’s so quiet—except for the noise of the waterfall—and peaceful. I feel like if I fell asleep, I could snooze for a hundred years and not be any different when I awoke. Time doesn’t touch this cave—or if it does, it touches it softly, slowly, subtly.

But then there’s the wretched terror, which is almost impossible to comprehend. And difficult to describe.

There’s a hole—the start of the tunnel—in one of the walls of the cave. And around and within it, the head and warped body of a man. The head hangs just above the hole, limp, its neck jutting out of the rock. Its body is spread out around it, mixed in with the rock, part of the wall. An arm far off to the left. A leg farther down to the right. The chest and stomach torn open, surrounding the hole, some inner organs visible inside the mouth of the tunnel.

At first, I think it’s a trick of the rock formation, that the head has been stuck there to emphasise the strange nature of the hole. Then I think that the body just adorns the outside of the rock, that bits and pieces have been stuck on or crammed into cracks. But as we move closer, drawn to it in silent fascination and horror, I see that isn’t right either.

The body is the rock. Somehow the two exist together, occupying the same space. It’s as if the rock melted and the man stepped into it, coming apart as the rock grew hard again around him. It must have been a painful way to die. Was he sacrificed? Did demons melt the rock and then—

The head bobs up and its eyes flicker open. I stifle a scream. There are gasps all around me. Goll, Lorcan and Connla raise their weapons automatically.

“No,” Drust says, signalling for calm. “It’s all right. He can’t harm us.”

“Don’t be… so… sure,” the man in the rock croaks.

“Balor’s eye!” Goll exclaims. “It speaks!”

“What is it?” Lorcan asks. “What manner of…?” He stops, eyes narrowing. Takes a step ahead of everyone, gazes at the face for a long moment, then looks back at Drust. “Druid, what spell is this? That face is yours!”

I don’t understand what he’s saying until I look again and see that the face hanging from the rock is very similar to Drust’s. Stubbly hair. Agonised eyes. A fuller beard. But his shape, his mouth, his expression.

“His name was Brude,” Drust says quietly, eyes locked with the man’s. “My twin brother. A druid like me.”

“Brotherrrrrr,” the man who once was—or still is—Brude sighs, then chuckles creakily. “You have… come… to witness… the glory?”

“Brude hated Christians more than most,” Drust says, ignoring the question. “I was never sure why.”

“Because… they… corrupt,” Brude hisses, eyes filling with fury. “They… change… that which… should not… be changed. They… destroy.”

“He decided to fight them,” Drust continues. “He sought a way to defeat them. Magic failed him. So did brute force when he tried to organise an army to lead against them. In the end he resorted to…” He trails off into silence for a moment, then speaks again, louder this time. “He opened the tunnel between our world and the Demonata’s. Invited the demons to cross. He’s responsible for all the savagery and deaths. He’s the one we must stop if we are to close the—”

“That’s why you came!” I cry suddenly. “The other druids refused to help, but your twin was the cause of the invasion. You felt guilty. You couldn’t bear to let so many people die because of him.”

Drust nods slowly. “We were like two parts of the same person when we were children. If he cut himself, I hurt. When I was happy, he laughed. That changed with time, but the bond was always there, linking us, binding us. What he’s doing is wrong. Christianity can’t be fought—and even if it can, it should be fought by human means, not demonic. I couldn’t stand by and let my brother—my own flesh and blood—commit such an atrocity against the entire human race. I had to stop him.”

“Not such a noble cause then,” Connla snickers. “You didn’t rush to our rescue because you cared for us, but because you didn’t like what your twin was up to.”

Drust shrugs. “Do my motives matter? I came. I wish to put a stop to the madness. That should be enough.”

“Can’t… stop,” Brude growls. Now that I’m closer I can see his heart, beating slowly within the wall, the rock pulsing along with it. So he’s not just alive within the rock—the rock is alive too.

“It has to stop,” Drust says. “This is wrong, Brude. The Demonata will destroy everything. They won’t stay on this island—they’ll find a way to cross the sea and spread throughout the world, killing all in their path.”

“Good,” Brude gurgles. “I want… them to. Except… our kind. The druids will… stand firm. We won’t… fall. The weak… will perish. The strong… will remain. The way it… should be.”

Drust shakes his head. “Even the druids would fall in the end. The Demonata don’t share, or even rule. They consume. All would fall to them—human, priestess, druid. All.”

Brude sneers. “If so… so be it. Better a world… of demons… than one… of Christian stain.”

“This is pointless,” Goll grunts. “We could stand here arguing forever and not get anywhere. Will I chop his head off at the neck and have done with it?”

“That won’t stop him,” Drust says, moving closer, breaking eye contact with his brother to motion me forward. “Brude’s spirit is infused with the rock. He has become part of the tunnel between worlds. He is beyond physical harm. We can only kill him by closing the tunnel.”

“Then do it, quick, and let’s be out of here,” Lorcan says, eyeing Brude uneasily, tugging nervously at his earrings, one after the other.

“You are a… twin too,” Brude says bitterly. “I can… tell. What would you… think if… your brother… spoke of killing… you?”

“If I was in your place, I’d say he had every right to spill my blood,” Lorcan answers stiffly.

“You lie,” Brude snarls. “Twin should… never raise a hand… against twin.” His snarl turns to a smile. “But… in this case… I don’t think… it will come to… that. I smell… a friend… among my… foes. He will… protect me.”

Goll frowns. “What’s he talking about?”

“Ignore him,” Drust mutters. “He’s mad. Let’s push on and—”

A cry of pain stops him. It’s Lorcan. As I whirl, the teenage warrior falls to the ground, clutching his chest, blood pouring out around his fingers.

“Demons!” Goll shouts, turning sharply, sword raised. He stops, bewildered. There are no demons in the cave behind Lorcan. Only Connla—with a blood-red knife and a killer’s smile.

Before anyone can react, Connla races to the cave entrance and roars up the shaft, “Demonata! Hurry to my side! There are enemies in your midst!”

Goll curses vilely and starts across the cave. But then we hear the sounds of demons pouring into the hole above and scrambling down the shaft. Goll stops, not sure what to do.

Drust ignores the chaos above us. He steps up, so he’s almost face to face with his twin, then speaks to me from the side of his mouth. “I’m going to start the spells. When I complete the first one, we’ll be able to enter the tunnel, where I’ll finish the rest.”