He turned to a maintenance access hatch, situated at the side of the alcove to the left of the blast doors. The hatch swung open noiselessly, revealing a cramped space lit with dim, red utility lighting and crowded with high-voltage conduits and data trunks. A narrow set of metal rungs led upwards and downwards into darkness. Before he'd left Aldurukh, Zahariel had memorised a circuitous route through the arcology's maze of accessways that would give him the best chance of reaching the rendezvous point unobserved. He'd need every bit of those six hours to make it to the meeting on time.

The Librarian stooped his shoulders and squeezed his way into the human-sized space, then pulled the hatch shut behind him. Darkness closed in on all sides, heavy with the scent of lubricants, ozone and recycled air. The hum of distant machinery reverberated through his bones.

With a deep breath, Zahariel began his descent into the depths.

Six hours and ten minutes later, Zahariel was crouched in the shadows at the mouth of a maintenance access corridor. Just a few steps away, a metal catwalk ran along the high wall of one of the arcology's many generator substations. From where he crouched he had a good view of the rendezvous point on the generator floor, six metres below. Something was wrong.

The time for the rendezvous had come and gone, and the rebel leaders were nowhere in sight. Instead, Zahariel saw a pair of men in utility coveralls waiting at the designated spot. One man puffed worriedly at a clay pipe, while the other tried to calm himself by cleaning his grimy nails with the point of a small knife. They looked like just another pair of generator techs stealing a few minutes' break away from the watchful eyes of their boss - except for the cut-down las-carbines hanging from their shoulders.

What had happened to Sar Daviel and the rest? Why had these two men been sent in their stead? Now, after ten minutes, the men were growing restless. No doubt they were coming to the conclusion that he wasn't going to appear either.

Zahariel gritted his teeth in irritation. He could let the men leave and try to follow them back to their superiors, but there was a significant risk that he could lose them in the arcology's labyrinthine passageways. That left him with only one viable option. The Librarian took a few, deep breaths, calling on his training to calm his mind and focus his thoughts, then he rose from concealment, took three quick steps and vaulted over the side of the catwalk.

He landed with scarcely a sound, not three metres away from the two rebels. The man with the knife let out a startled squawk and recoiled from the Astartes, his eyes widening in fear. The pipe-smoker whirled, following the other man's startled gaze. To his credit, he kept his composure much better than his companion.

'You're late,' the rebel said around the stem of his pipe.

'I didn't come here to meet with you,' Zahariel said coldly. 'Where is Sar Daviel?'

The two rebels exchanged nervous glances. 'We're supposed to take you to him,' the pipe-smoker said.

'That wasn't what we agreed upon,' Zahariel said, a shade of menace creeping into his voice. The knife-wielder blanched, his grip tightening on the handle of his tiny penknife. If the situation hadn't been so serious, the Librarian might have been tempted to laugh.

The other rebel plucked the pipe from his lips and gave a disinterested shrug. 'Just doing what we're told,' he said. 'If you mean to parley, then follow us. If not, well, I expert you know the way out.'

'Very well,' the Astartes said coldly. 'Let's go.'

'First things first,' the pipe-wielder said. He reached into a pocket of his coveralls and drew out a small auspex unit. Placing the pipe back in his mouth, he activated the unit and adjusted its settings, then swept it over Zahariel from head to toe.

Zahariel felt his choler rise as the rebel performed his scan. 'The agreement was that I not come armed or armoured,' he said, biting off each word.

The rebel was unperturbed. 'That's as may be. I still have my orders.' Finished with the scan, he checked the unit's readout, then nodded to his companion. 'He's clear.'

The second rebel nodded, then put away his penknife and started off towards the mouth of a dimly-lit corridor on the far side of the generator room.

'Follow him,' the pipe-wielder said. 'I'll be right behind you.'

Biting back his anger, Zahariel fell into step behind the lead rebel.

They walked for more than an hour, following a long, torturous route through the maintenance spaces that would have completely disorientated a normal man. As it was, Zahariel had only a vague notion of where in the arcology they were. He was certain that they had descended through another two sub-levels, making them at least a hundred metres below ground.

At the end of the trek Zahariel found himself walking down a long, dark corridor that seemed to go on for at least a kilometre. After several minutes he began to see a faint, grey luminescence up ahead. He smelled brackish water and wet stone, and a low, hissing sound filled his ears. Soon the grey light resolved itself into a doorway that opened onto a clattering metal catwalk suspended over a man-made waterfall. To the right of the catwalk, close enough to touch, was a wall of plunging water that churned into foam just two metres below Zahariel's feet before passing under the catwalk and through a metal grate off to his left. They had reached one of the arcology's many wastewater purification plants, Zahariel realised. At the far end of the catwalk, about fifty metres away, a small, permacrete blockhouse jutted from the chamber wall. Two armed rebels stood outside the blockhouse door, their hands nervously gripping their stolen lasguns.

The guards halted them at the end of the catwalk and conferred with Zahariel's guides in low, urgent tones; he tried to listen in on what was being said, but the white noise of the waterfall made it impossible. After a brief exchange, the guards nodded and stepped to one side. The pipe-wielding rebel turned back to Zahariel and gestured to the door with a nod of his head. 'They're waiting for you inside,' he said.

At once, Zahariel's anger began to rise. Without a word, he rushed past the four men, pushing open the door with the flat of his hand and storming inside. He found himself in a small room, perhaps five metres to a side, which was lined with banks of controls and flickering data-plates. Four rebel soldiers stood in a tight knot on the opposite side of the room, close to a featureless metal door. To his left, Zahariel saw Lord Thuriel and Lord Malchial sitting in a pair of the control room's utilitarian chairs. Malchial was clearly agitated, leaning forward in the chair with his hands clenched so tightly his knuckles were white as chalk. Thuriel, on the other hand, was at ease, peering at the Librarian over steepled fingers. His dark eyes held nothing but contempt.

'So you chose to come after all,' Thuriel sneered. 'I'd half given up on you.'

'Had you been at the agreed-upon place you wouldn't have had to wait,' Zahariel shot back. 'We haven't the time for games, Lord Thuriel. Where are Lady Alera and Sar Daviel?'

'That's none of your concern,' Thuriel said. He turned slightly and nodded to the men at the door. As one, the four rebels turned to face Zahariel, raising their weapons. Two of the men were armed with heavy, blunt-nosed plasma guns. For a moment Zahariel could only stare at the rebels. The idea of violating the time-honoured tradition of parley shocked him more profoundly than any warp-spawned horror could.

'Upon further consideration, we've decided to make you our guest,' Thuriel said with a cruel smile. 'I think a high-value hostage will persuade Luther to take our demands seriously.'