'Fuel for the fire,' Luther said, half to himself. 'The sorcerers are using the violence and bloodshed to weaken the barrier between the physical world and the warp and facilitate their master ritual.' He cast a meaningful glance at Lord Cypher, who nodded.

Zahariel scowled at the secret exchange, wondering what secrets Luther had uncovered from the forbidden library. 'Then we have to find a way to strike directly at the sorcerers and their ritual,' he declared.

'If we can locate them in time,' Luther said grimly. 'The ritual must be close to completion at this point.'

'Zahariel can lead us there,' Cypher said. His hooded head swivelled to regard the Librarian. 'You can sense the turbulence in the warp generated by the ritual, can you not?'

'I…' Zahariel paused, glancing from Lord Cypher to Luther. The Master of Caliban was staring at him expectantly. Was he being manoeuvred into something? Israfael's stricken face hovered before his mind's eye like a ghost. He shook his head, as though to clear it. 'That is, yes, I can, but that kind of prolonged exposure to warp energy is not without risk.'

Luther grinned wryly. 'Brother, believe me when I tell you that if we don't stop this ritual we're all going to be exposed to more warp energy than is really healthy.'

A strange, wheezing note blurted from Attias's vox grille. Zahariel turned to stare at the skull-faced Astartes. The sound continued, and it took the Librarian a few moments to realise that Attias was laughing. Cypher started to chuckle, and then Zahariel couldn't help but join in as well, dispelling the tension of the moment.

'Well, brother?' Luther prompted.

Zahariel bowed his head. 'Give me a moment to centre myself,' he said, clenching the force staff tightly and focusing his awareness through his armour's psychic hood.

At once he felt the churning maelstrom of the warp whirling about him. Its energy licked at him like tongues of flame, trying to find purchase in his soul. Jagged slivers of ice dug painfully into the back of his skull as the hood tried to shield him from the storm.

The whirlwind spun about him, drawing him downward towards its locus like a gaping maw. Something lay at its centre, he sensed; a seed of darkness, hungry and impatient for release.

Zahariel staggered slightly at the vertiginous pull of the ritual, holding himself apart from it by sheer effort of will. 'I can feel it,' he gasped. 'The sorcerers are trying to open a path for something to come through. Like Sarosh, only… worse, somehow.'

'Can you lead us to them?' Luther said.

Zahariel concentrated on the vortex, following its currents with his mind. The biting cold in his head increased. Frost spread along the force staff's metal shaft. 'The locus is deep within the earth,' he said with a grimace. 'I'll be able to refine its position more precisely as we go.'

'Excellent,' Luther said. 'We'll have Hadziel unlock a bank of maintenance lifts that will take us directly to the lowest sub-level, then fight our way to the locus from there.'

The Master of Caliban spun on his heel, snapping orders to Colonel Hadziel and to the other three squad leaders waiting at the landing pad. With an effort, Zahariel tried to re-orient himself in the physical world once more. The transition was much more difficult than he expected; even with the buffer provided by the psychic hood, the energies of the maelstrom still plucked at him, as though it had sunk barbs deep into his soul. He felt strangely numbed, unmoored within his own skin, and he knew that the grip of the storm would only grow stronger the closer he came to the centre of the ritual.

He blinked, trying to clear his eyes, and found Lord Cypher studying him speculatively. Before Zahariel could ask what he was staring at, the enigmatic Astartes abruptly turned away.

They descended into darkness, lit only by feeble red emergency lighting inside the maintenance lift's metal cage. Hadziel had authorised the activation of a bank of four lifts that would allow Luther's four assault squads to deploy together, concentrating their strength against whatever foes awaited them. Based on their experience at Sigma Five-One-Seven, Zahariel had advised choosing the set of lifts in the closest proximity to the arcology's main thermal core.

The strength of the maelstrom increased steadily the deeper they went, until Zahariel scarcely had to focus his awareness in order to sense it. The unnatural energies sank effortlessly through his armour and pulsed sickeningly against his skin. Frost coated the housing of his psychic hood and sent needles of icy feedback into his brain. The storm winds tugged ruthlessly at him, tearing at his mind and soul with increasing vigour.

Finally the lift jerked roughly to a halt, two hundred metres below the earth. They'd reached the lowest sub-level of the arcology. Luther gave a nod to the Astartes manning the controls, and the lift doors clattered open, revealing a broad, low-ceilinged chamber formed of fused permacrete. The air was stiflingly humid and thick with the stench of corruption.

Here, as with the lower levels at Sigma Five-One-Seven, the earth had already begun to reclaim the space. Glossy greenish-black vines sprouted from cracks in the walls and along the floor, and a dripping, greenish mould covered much of the ceiling. Inserts chittered and squirmed through the tainted growth, or droned through the thick air on blurring wings. Sickly blue luminescence radiated from colonies of fungus that sprouted in haphazard clusters overhead, providing ample light to the Astartes' enhanced night vision.

The Dark Angel squads deployed swiftly from the adjoining lifts. Three assault squads took the lead, forming a protective arc in front of Luther and the command squad, and orienting their weapons on the three entryways on the opposite side of the chamber. Two men in each of the assault squads carried a hand flamer, while two of the veterans in Luther's command squad were armed with powerful, short range meltaguns. The rest carried roaring chainswords and blunt-nosed bolt pistols, ideal for the kind of close-quarters fighting they expected to encounter. They were forty strong, a fearsome display of force. Entire worlds had been brought into compliance with less.

Luther led the command squad into the chamber. His huge sword Nightfall burned a fierce blue in his right hand, and his ornate bolt pistol gleamed dully in his left. Zahariel stood next to him, clutching his force staff with both hands, while Brother Attias and Lord Cypher brought up the rear. Cypher held his plasma pistol ready in his right hand. The leather-bound grimoire was clutched tightly against his chest.

The Master of Caliban leaned close to Zahariel. 'Can you sense the ritual in process?' he asked quietly.

Gritting his teeth, Zahariel focused his awareness through the psychic hood. The dampener was already straining at the limit of its abilities; he could smell the strange mix of overheated circuitry and frozen metal. This close, he could sense rhythms pulsing through the howling psychic wind, like discordant notes struck by a madman's hand. The vibrations represented the symbolic chants that coaxed the energies of the warp into the physical realm.

'The ritual is well advanced,' the Librarian said, suppressing a groan of disgust. 'It could reach its climax at any time. We have to hurry!'

Luther nodded. His dark eyes shone with fevered intensity. 'Listen, Zahariel. When we reach the ritual site, I want you to keep close to me. We have to confront this entity, together. I have the knowledge, but I lack the ability to manipulate the forces of the warp.'

Zahariel shook his head. 'Confront it? You mean drive it back.'

'No,' Luther said. 'At least, not yet.' He turned and nodded at the grimoire that Cypher carried. 'That book contains the means to subjugate the spirit, bend it to our will. If we can reach it at the right moment, while it's still weak.'