'The time for dissembling is past,' Zahariel said, his tone as cold and sharp as ice. 'Take us to Luther. Now.'

Cypher gritted his teeth under the psychic onslaught. 'I won't…'

'Then I'll dig his location out of your brain,' Zahariel said, 'along with any other secrets you've been keeping. I can't say there will be much left of you afterwards, though.'

Zahariel drove his probe deeper into Cypher's mind. The Astartes went rigid. A thin trickle of blood seeped from one nostril.

'Stop!' Cypher said in a choked whisper. 'I'll do it! I'll take you to him! Just—'

He slumped with a groan as Zahariel released him. Cypher's head drooped for a moment, his shoulders heaving. When he looked up at the Librarian, his expression was savage.

'You don't know what you're trifling with, you fool,' Cypher snarled. 'The primarch—'

'The primarch isn't here,' Zahariel said coldly. 'So I'll trifle with whatever I must. Now get up. We haven't any more time to waste.'

Cypher got up from behind the desk without another word. They followed him from the room, hovering at his shoulder like ravens.

Cypher led them into darkness, deep within the bowels of the Rock.

From the Circle Chamber, they descended through a secret stairway at the top of the Grand Master's dais that Zahariel never knew existed, yet at the same time seemed tantalisingly familiar. Try as he might, he couldn't reconcile the two notions; the more he concentrated, the more his head began to ache. Finally, he decided to let the matter go rather than compromise his already frayed concentration. The pain in his skull subsided, but didn't entirely vanish.

The stairwell ended at a low-ceilinged room that might once have been a meeting space in times past; now the ancient brickwork was pierced by modern archways of fused permacrete that continued even further into the depths. Cypher led them through the dimly-lit passageways without hesitation, threading his way through a labyrinth of tunnels that began to tax even Zahariel's genetically-enhanced memory. Deeper and deeper they went, down into the very heart of the mountain, until it felt as though they had been walking for hours. Zahariel reckoned they were more than a thousand metres down when Cypher turned down a narrow, vaulted corridor that abruptly ended at a tall, arched doorway. The doors themselves, Zahariel noted with surprise, were plated with adamantium, and set in a reinforced frame. Anything powerful enough to breach that portal would also incinerate anything on the other side, his trained mind noted.

Standing before the doors, Cypher dug a sophisticated electronic key from within his robes. With a last, furious glance at Zahariel, he held the key up to the portal and touched the actuator. Bolts drew back into the frame with an oiled clatter, and the tall doors swung silently inward.

The library within was built vertically, its packed shelves rising on eight sides to a vaulted ceiling fifty metres overhead. Long, thin lumen strips set into the stone at the corners of the eight walls filled the space with pellucid light. The air smelled faintly of ozone and machine oil. High up along the walls Zahariel could see four small logo-servitors waiting unobtrusively in the shadows, clinging to the walls with their spindly limbs and watching the Astartes with small, red eyes.

Zahariel reckoned the floor of the library was perhaps thirty paces across, covered with thick rugs to combat the subterranean chill. Reading desks and heavy wooden tables were arrayed haphazardly about the room, piled with open books and ancient, musty scrolls. More books were scattered in drifts across the floor, between and beneath the tables. There were so many that the Astartes were forced to pause just beyond the threshold, afraid of treading upon the fragile tomes.

The air in the library was utterly still, heavy with the dust of ages. The only sound Zahariel could hear was the soft whirring of servo-motors overhead. A current of invisible energy, faint but palpable, sent tendrils of ice spreading through his skull.

He drew a breath and spoke into the cathedral silence. 'Luther? My lord, are you here?'

A figure stirred in the shadowy depths of a high-backed chair near the centre of the room. Zahariel could just make out the head and shoulders of a man, limned in the faint, bluish-silver light.

'Zahariel,' Luther replied. His voice was rough, as if from long hours of exertion. 'You shouldn't be here.'

Lord Cypher took a cautious step forward, distancing himself from the rest of the Astartes. 'I beg your forgiveness, my lord,' he said with bowed head. 'They would not honour your wishes.'

Zahariel glared at Cypher's back. 'This has nothing to do with anyone's wishes,' he snapped. 'This is a time of crisis. Caliban stands upon the brink of disaster, my lord. The Legion must act, now, or all is lost.'

Luther rose slowly from the chair and stepped forward into the light. His eyes were sunken and his cheeks hollowed, as though from the ravages of a terrible illness, and there were dark ink marks on his hands, wrists and throat. The Master of Caliban paused, his cracked lips working as he peered at the figures standing at Zahariel's shoulder.

'Master Remiel?' he said. 'Is this a dream? I thought you long dead.'

'I continue to confound my enemies, my lord,' Remiel answered with a faint smile.

'I'm glad to hear it,' Luther said. His expression turned sombre. 'But I see you travel in the company of rebels these days,' he said, pointing to Sar Daviel. 'Is it me you seek to confound now, master?'

Remiel didn't flinch from the accusation. 'No loyal son of Caliban is an enemy of mine,' he answered coolly.

Zahariel studied Luther with concern. 'My lord, when did you last eat or drink?' he asked. Though an Astartes could go for many weeks with minimal nourishment, he knew that Luther's body hadn't received the full suite of metabolic enhancements. By the look of things, Zahariel feared that he'd been fasting for weeks.

The Master of Caliban ignored the question. 'What is going on here, brothers?' he asked, his voice regaining some of its strength and authority.

'The truth has become known,' Israfael said grimly. 'Rumours have spread through the Northwilds that the Imperium is in league with sorcerers,' he spat angrily. 'Riots have broken out, and the Administratum is up in arms.'

Luther's eyes widened in anger. 'How did these rumours start?' he demanded. 'I ordered this knowledge kept secret! Who is responsible?'

Zahariel took a deep breath and stepped forward. 'I am,' he said gravely. 'The fault is mine.'

The admission took Luther aback. 'You?' he said disbelievingly. 'But why?'

All eyes turned to Zahariel. Head high, the Librarian reported everything he'd seen and done at the arcology. Luther listened, his expression growing harder by the moment. He gave no reaction to the proposed truce with the rebels, though both Astelan and Israfael glowered angrily at the news.

Zahariel concluded by relating what they'd recently heard from the Northwilds. 'Things are balanced on a knife's edge, my lord,' he said. 'If we strike quickly, we might still be able to contain the situation.'

'No, we can't,' Luther said flatly. He shook his head, his expression bleak. 'It's far too late for that. I don't fault you for what you did brother, but there's no going back now. Caliban's fate is sealed.'

Luther turned in the stunned silence that followed and walked to one of the heavy reading tables. He bent over a massive, leather-bound tome, brushing the tips of his fingers across one of the thick, vellum pages. Zahariel caught a better glimpse of Luther's hands, and saw that the ink marks there were actually symbols of some kind, laid out in a geometric pattern. A chill raced up the back of his neck.