'That's where the thermal vent is located,' the chapter master said.

Zahariel could see the memories of Sarosh lurking deep in Astelan's eyes. They all remembered the vast cavern beneath the earth, filled with millions upon millions of corpses offered up to the Saroshi's obscene god.

Not here, he wanted to say. This is Caliban. Such things do not happen here.

Instead, Zahariel gripped his force staff tightly in his hand and addressed the chapter master. 'Assemble the squad,' he said, his voice betraying nothing of the despair he felt.

Astelan nodded curtly. 'What are your orders?'

Zahariel glanced once more at the dark pict screens. 'We're going to go down there and find out who is responsible for this,' the Librarian replied.

'Then, by the primarch, they're going to pay for what they've done.'

They formed up by the Land Raider as the sun was setting behind the mountains to the west. A thick bank of grey clouds was rolling ponderously towards the site from the south, carrying with it the threat of a storm. The weather had grown increasingly wild and unpredictable over the years as the Imperium transformed the surface of the planet and filled the skies with plumes of smoke from their manufactories. Magos Bosk and the rest of the Administratum insisted that the changes were nothing to be concerned about. Zahariel eyed the looming clouds warily and wondered if Magos Bosk had ever conducted a squad-level skirmish in a raging gale. He confessed to himself that the odds seemed unlikely in the extreme.

They boarded the assault tank and crossed the wide landing field, heading into the deep shadows filling up the alleys and access ways to the east of the site. The plant's massive thermal exchange unit was a black tower - wider at the base, then narrowing a bit at the middle before flaring open once more as it soared high into the sky over Sigma Five-One-Seven. Red and blue hazard lights flashed insistently along its length, warning low-flying aircraft to keep away; when the plant went into full operation the tower would be wreathed in hissing ribbons of waste steam, tinted a sickly orange by chemical flood lamps.

The Land Raider's driver circled around the base of the huge tower until he came upon a wide, low-ceiling entrance at the southeast side. At Zahariel's command, the tank rumbled to a halt a few dozen metres from the opening, then the squad dismounted into the garnering darkness. Immediately, Astelan pointed to three sets of cargo crates, each arrayed in a crescent shape with the closed ends pointed towards the tower entrance. Zahariel recognised them even before he saw the familiar shapes of heavy stubbers aimed at the thermal unit's entrance.

The Astartes approached the makeshift weapons cautiously, sweeping the shadows with their bolt pistols. Dried blood stained the permacrete around each of the positions; Zahariel's keen eyes detected scores of small craters where lasgun bolts had eaten into the pavement around the emplacements. A bloodstained portable vox-unit lay near the centre weapons station, its control panel smashed to pieces.

Zahariel eyed the heavy stubbers. None of them showed signs of having been fired. 'It looks like the reaction force tried to set up a security cordon around the thermal plant's entrance,' he declared, 'the gunners must have been ambushed later, once the others were gone.'

Astelan nodded in agreement. 'You think they realised what was going on?'

The Librarian shook his head. 'They knew only what the enemy told them,' Zahariel said. 'I expect the company commander got off his Condor and found a frantic man or woman in labourer's coveralls who told him that the rebels had taken over the thermal unit and were planning to blow it up. So the captain rushed in there with everything he had, hoping to stop the enemy before it was too late.'

Astelan glanced back at the Librarian. 'And now we're going in there as well?'

Zahariel nodded grimly, raising his force staff. 'Whatever the enemy might expect, they aren't ready for the likes of us.'

The members of the squad readied their weapons in mute agreement. Attias moved up alongside Zahariel, his silver death's-head mask seeming to float eerily out of the darkness. 'Loyalty and honour,' he rasped.

'Loyalty and honour, brothers,' Zahariel answered back, and led his squad inside.

The air inside the thermal exchange unit was hot and humid, gusting like the breath of a huge, hungry beast. Red emergency lighting bathed the interior crimson, outlining billowing clouds of steam and glistening on drops of condensate flowing from overhead pipes and ductwork. Zahariel smelled the bitter reek of corroded metal and freshly spilled blood.

'I thought the thermal exchanger wasn't online yet,' he said aloud.

'It's not,' Gideon replied. 'I checked the readouts myself.' He pulled his auspex unit from his belt and tested it. The screen flickered and then filled with a cascade of data. The Astartes tried several different detection modes, then shook his head in disgust and put the unit away. 'No readings,' he reported, 'or at least, none that make any sense. I'm picking up a lot of interference from somewhere close by.'

'Somewhere,' Attias echoed, 'or something.'

'Tactical pattern Epsilon,' Zahariel interjected curtly, unwilling to let that train of speculation proceed any further. 'Stay sharp, and watch for likely ambush points.'

Within moments the squad was arrayed in a rough octagonal formation, with a warrior at each corner of the octagon and Zahariel and Gideon, the auspex bearer, in the centre. It was a solid formation that drew on the ancient teachings of the Order, and was suited to dealing with close assaults from any direction. Abruptly he found himself wishing that he'd thought to equip the squad with a flamer or two before leaving Aldurukh, but that couldn't be helped now. Once he was satisfied that all of his warriors were in position, Zahariel waved the squad forward.

Drawing on the maps he'd memorised, Zahariel guided the squad through the twisting corridors surrounding the base of the thermal tower. Visibility was limited; even with the Astartes' enhanced senses the plumes of mist and the dim red lighting created illusory patterns of movement and obscured vision beyond more than two metres. Zahariel could not help but admire the courage of the Jaegers who had preceded them; the human troops would have been all but blind as they tried to reach the lower levels of the tower. He doubted that they'd made it very far.

The terrible heat and the reek of corruption increased as they pressed further inside, and the sense of malevolence grew stronger and more focused on Zahariel and the squad. He could feel its weight pressing against him like a smothering cloud, probing his armour in search of a way inside. The cables connecting his mind to the psychic hood grew deathly cold, and a film of black frost condensed on the haft of his force staff despite the cloying heat. He was tempted - strongly tempted - to reach out with his own psychic power and get a sense of the enemy that lay somewhere ahead, but years of training with Brother-Librarian Israfael cautioned against it. Don't waste your energies swinging blind, Israfael had told him many times. Or worse, leave yourself open to a surprise attack. Conserve your strength, maintain your defences, and wait for the enemy to reveal themselves. And so he did, resolutely pushing the squad forward and waiting for the first blows to fall.

There were four industrial-grade lifts that provided access to the tower's lower levels, but they were deathtraps as far as Zahariel was concerned. If the enemy had access to a meltagun - and the Jaeger reaction force had carried two - then a single blast into such a tight space could wipe out half his squad. He had Brother Gideon disable their controls so the enemy couldn't use them either, then they began their descent via one of the tower's four long stairways.