'You can't be serious!' Zahariel cried. 'What you're talking about is madness! The Emperor—'

Luther stepped close, until he was nearly whispering in Zahariel's ear. 'Yes. The Emperor has forbidden this. Why? Because he fears the beings of the warp. That's something we must learn to exploit, if Caliban is to remain free.' He looked deeply into Zahariel's eyes. 'Do you trust me, brother?'

Zahariel found himself nodding, despite the misgivings in his heart. 'Yes. Of course.'

'Then help me. It's the only way.'

Without waiting to hear Zahariel's reply, Luther turned and waved the assault squads towards the rightmost of the three large openings on the other side of the landing. So far, the path to the ritual site seemed to lead to the arcology's primary thermal core, just as it had at Sigma Five-One-Seven. With a pair of flamer-wielding Astartes in the lead, the first assault squad advanced into the broad, vine-choked passageway. Luther's command squad was third in line, with the last assault squad covering the rear.

The corpses came at them from three sides. A few hundred metres down the passageway, it was bisected by another pair of wide corridors. The enemy, showing a rudimentary grasp of tactics, allowed the first and second squads to pass this junction before triggering their ambush. With scarcely a sound, hundreds of rotting corpses shambled out of the darkness, attacking the head of the advancing strike force and trying to drive into its midst from either side.

Flamers hissed, filling the passageways with streams of searing promethium. Bolt pistols barked on every side, felling the advancing creatures with well-placed shots to the head. The Astartes continued to fire even as the corpses surrounded them, drawing into arm's reach and trying to drag down the armoured warriors by sheer weight of numbers. Chainswords roared and slashed, severing limbs and splitting torsos.

The Dark Angels stood shoulder to shoulder in the confined space, never yielding a centimetre to the unearthly horde. At the centre of the formation, standing at the junction of the passageways, Luther roared encouragement to his warriors and put down one corpse after another with his pistol. Zahariel and Attias joined in with their own pistols, adding to the whirlwind of steel that took a fearful toll of the enemy.

For several long minutes the battle raged against the walking dead. The corpses pressed harder and harder against the Astartes - and then, inevitably, the pressure began to wane. The strike force, sensing that they had absorbed the brunt of the attack, began to press further down the passageway. Flamers continued to hiss and spit, until the walls of the passage shimmered with heat and the air grew thick with smoke and the stench of burnt meat.

Zahariel followed Luther through a waking nightmare. They advanced in the wake of the lead assault squads, moving down a tunnel of burning vines and shredded bodies. The slaughter was incredible; within only a hundred metres the Librarian found himself walking on a literal carpet of broken bodies. In places his boots sank into piles of blood and bone that rose nearly to his knees.

The Astartes drove inexorably forward, grinding the enemy beneath their heel. Then, without warning, the passageway widened into a huge chamber that crackled with unnatural energies. They had reached the thermal core.

Blasting their way through a faltering rear guard of corpses, the first and second assault squads broke through into the chamber far enough to make room for Luther's command squad. Then they halted, weapons ready, waiting for word from their commander.

Luther and Zahariel emerged into the cavernous room with the rest of the command squad close behind. Ahead, arcs of violet lightning leapt from the monolithic bulk of the thermal core and etched looping scars across the permacrete floor. The air stank of ozone and the sickly-sweet reek of decaying flesh; it rippled invisibly against the skin, churned by unnatural energies that radiated from the vast ritual circle at the centre of the space.

A half-dozen queen worms were curled about the outside of the circle, their segmented bodies writhing frenetically in response to the building intensity of the ritual. Their mandibles clashed and their multiple eyes glowed with a power of their own as they drove thousands of corpses against the arcology's hard-pressed defenders.

Just beyond the worms, standing at precisely-determined points along the perimeter of the ritual circle, stood the sorcerers. The Terrans were clad in torn and stained robes that had been painted in arcane sigils that shone with a strange, pellucid light. Zahariel saw that their skin was waxy and mottled in shades of black and grey, as though they were little more than corpses themselves. Their heads turned fearfully at the arrival of the Astartes, but their leader, a towering figure with his back to the Dark Angels, rallied them with clenched fists and shrieked curses until they resumed their efforts.

At the centre of the circle, Zahariel could just make out massive coils of scaly hide, larger by far than the queen worm that had nearly slain him and his squad at Sigma Five-One-Seven.

Zahariel felt a surge of power in the great chamber that seemed to rise up from deep within the earth. Black vapours, reeking of sulphur and rot, rose in a flood from the deep pit where the thermal core was set. The ritual was reaching its culmination.

'We're nearly out of time!' he cried out.

Luther heard and nodded grimly. He raised his glowing sword. 'For Caliban, brothers!' he cried, his voice echoing like a trumpet call over the cacophony of the ritual chamber.

'For Caliban!' the Astartes answered. 'For Luther!' As one, they charged forward.

The queen worms outside the circle reacted at once, whipping about and screeching their fury, but they were caught in a veritable storm of bolt pistol fire, searing flame, and the fearsome blasts of meltaguns. Mass-reactive rounds punched through thick layers of scale and detonated in the soft flesh beneath, blasting gory craters in the worms' flanks. Two of the creatures thrashed and hissed, bathed in streams of fiery promethium. A third blew apart as a pair of meltagun shots struck in at the head and midsection, showering the rest with splashes of steaming ichor.

Yet despite their terrible wounds, the surviving worm queens fought on. Two of the creatures focused on Luther, their mandibles clashing as they lunged at the knight from the left and right. Zahariel saw it unfold, and thought of Brother Gideon, his body shorn in half by a worm's scissor-like bite.

But Luther was a born warrior, a man who had been fighting the monsters of Caliban all his life. As the monsters lunged, he ducked low and to the left, bringing up his power sword as the worm's leap carried it just past his right shoulder. Nightfall pierced the side of the worm's head, just behind the mandible, and like a claw it tore a burning gash more than halfway along the worm queen's length. The second worm found its attack blocked by the first creature's lunge, causing it to check its thrust and slide, snapping, over the mortally-wounded queen's back. Luther saw it coming and put out one of its eyes with an explosive bolt from his pistol. A plasma shot from Lord Cypher struck the opposite side of the queen's skull a moment later, leaving a glowing crater gouged into the bone and boiling its brains in the blink of an eye.

Brother Attias fell upon the mortally-wounded queen and began to saw its head off with his roaring chainsword. To Zahariel's left, a burning worm leapt into the midst of one of the attack squads, flattening them beneath its bulk and madly snapping at armoured limbs and torsos. Another worm, streaming ichor from scores of bolt-pistol wounds, snatched up a Dark Angel in its mandibles and lifted him high, crushing his armour plates like paper. The Librarian watched the warrior slap a krak grenade right between the monster's eyes, and both he and the worm's head disappeared in an angry yellow flash.