Nemiel moved silently forward and knelt down beside the Techmarine. 'What's wrong?' he whispered.

Askelon raised his chin slightly, like a hound tasting a scent. 'Faint surveyor pulse, emanating from farther down the tunnel,' he said. 'We're outside its extreme range.'

The Redemptor raised his bolt pistol. 'A sentry?'

'Yes,' Askelon replied. 'It's a sigma-sequence pulse, so it's not one of the small patrol units. Most likely it's a stationary unit, like a sentry gun.'

'Then it's probably sitting right at the feet of the ladder leading up to the foundry.' Nemiel said. 'Any way to outflank it?'

Askelon shook his head. 'Unlikely. But there might be a way to temporarily incapacitate it.'

'Tell me.'

The Techmarine pointed at the conduits lining the walls around them. 'This is category nine conduit; it's the most heavily-shielded insulator available,' he explained. 'But there's so much power going through these lines that there's still significant electromagnetic radiation leaking into the tunnel.'

'And how does that help us, exactly?'

'If I cut into the conduits I can use my armour's power plant to send a feedback surge down the line towards the sentry unit,' Askelon said. 'A powerful enough spike in electromagnetic radiation will overload its auspex receptors and force a reset. That will render it blind and unable to communicate for approximately thirty seconds.'

'Approximately?' Nemiel said.

'If I could see the type of sentry unit I could tell you down to the millisecond,' Askelon said. 'As it is, it could be one of a half-dozen models. Thirty seconds is my worst-case estimate.'

Nemiel nodded. 'Get to work.'

The Redemptor went back to the squad and told them what was happening while Askelon quickly marked out which conduits to tap and went to work. With deft movements he drew out a small, powerful plasma torch and cut open a half-dozen of the steel tubes, then he opened an access panel on the side of his backpack power unit and began attaching a number of heavy-gauge cables to the contacts inside.

Several minutes later, the Techmarine was ready. He glanced back at Nemiel, who gave him the nod to proceed. Askelon quickly attached the cables to the power lines inside the conduits. His armour stiffened abruptly. Immediately, Nemiel saw the Techmarine's status indicators begin flashing urgently on his helmet display. The core temp of his power unit spiked beyond allowable tolerances and continued to climb. Askelon's physio-monitors began to fluctuate as well, as feedback coursed through the suit's neuro-interfaces and into his body.

'There's smoke rising from his power plant,' Kohl whispered urgently.

'Let him finish!' Nemiel hissed. 'It's the only way.' Seconds passed. Nemiel watched Askelon's indicators pulse from green to amber, and then amber to red. Without warning, a fountain of sparks shot from the servo-arm housing between the Techmarine's shoulders. Askelon spasmed, throwing out his hands and shoving himself away from the power conduits. The Techmarine fell backwards, stiff-legged, and crashed into the far side of the tunnel.

Nemiel and the rest of the squad rushed to the downed Astartes. The air around Askelon shimmered with heat, radiating from his overloaded power unit. The Techmarine turned his head; squawks of sound crackled from his helmet's speaker. Nemiel didn't have to hear the words to know what Askelon was trying to say.

'He's sent the pulse,' Nemiel told the squad. 'Brother Marthes, take point. Sergeant Kohl, help me with Brother Askelon. Let's move!'

The Astartes sprang into action, charging down the tunnel behind Marthes, who advanced with his meltagun held ready. Kohl and Nemiel brought up the rear, dragging the limp form of Askelon between them.

Three hundred metres down the tunnel, the passageway fed into a large, square structure that echoed the permacrete blockhouse they'd entered at the manufactory. The plasteel rungs of another ladder climbed upward, presumably into the foundry's assembly building. Sitting at its feet, just as Nemiel suspected, crouched a matte-black sentry gun. Armed with a turret-mounted twin-linked lascannon, the automated unit crouched on four stubby legs like a hungry spider waiting for prey. Nemiel could hear the hum of its power unit as they approached. Its twin guns were aimed straight down the tunnel at the approaching Astartes. A single shot would cut through their armour like tissue.

'Up the ladder!' he ordered the squad. 'Get up and get out of sight!'

Marthes stepped around the sentry gun and began climbing at once. Vardus paused at the bottom rungs, his heavy bolter slung at his side. 'What about Askelon?' he said.

'We'll manage,' the Redemptor shot back. 'Now hurry, brother!'

Vardus started his climb, with Ephrial hot on his heels. Nemiel consulted his internal chrono: they had just twelve seconds left. He looked to Kohl as they reached the bottom of the ladder. 'We need to find a way to shut off the sentry gun,' he said. 'There must be an access panel—'

Askelon shook his head sharply; the ceramite edges of his helmet scraped against his gorget, suggesting he'd sustained damage to his armour's muscle fibres. 'No,' he said, his voice coming through his helmet's damaged speaker as a tortured croak. 'Can't risk it. I… I can climb.'

'All right,' Nemiel growled. 'You go first. Kohl, you're next. Help him as much as you can.' He would stay until the last moment; if they ran out of time, he would tear open one of the sentry gun's access panels and try to shut it down.

Askelon grabbed hold of the metal rungs and started climbing, seeming to gather strength with each lunge of his legs. Kohl was right behind him, ready to provide a judicious shove if the Techmarine faltered. Nemiel counted the seconds and checked the sentry gun for likely access points.

Vardus and Ephrial leaned over the hole, grabbed Askelon's folded servo-arm and hauled him bodily up into the chamber above. Kohl raced up behind him. 'Clear!' he hissed to Nemiel.

The Redemptor leapt for the rungs and scrambled upwards as quickly as he could. The timer on his display hit zero when he was halfway up. There was a series of rapid clicks and whirring sounds directly beneath him as the sentry gun sprang back to life.

Hands reached down and grabbed the edges of his pauldrons. Nemiel felt himself yanked upwards like a sack of grain and deposited roughly on the permacrete of the upper floor.

The Astartes froze, listening intently. Below them, the sentry gun clattered and whirred a moment more, then resumed its quiet vigil.

Nemiel looked over at Askelon's prone form. 'Any sign of alarm?'

The Techmarine reached slowly for his helmet and undid its clasps. Askelon pulled the helm away, revealing a sweat-streaked face stippled with broken blood vessels. A trickle of blood seeped from his nose and the corners of his eyes. 'No change,' he said in a husky voice. Blood slicked the Techmarine's teeth.

Nemiel rolled over and rose to his knees beside Askelon. 'How badly are you hurt?' he asked quietly.

Askelon chuckled faintly. 'I'm no Apothecary, brother,' he replied. 'The machinery of a living body is too complex even for me.' He levered himself to a sitting position with a grunt. 'Armour integrity is at sixty-five per cent. Power levels at forty per cent. Muscle fibre reflex is compromised, and I think I melted the motors on my servo-arm.'

Nemiel frowned. 'You didn't mention that tapping those conduits would likely kill you,' he growled.

The Techmarine managed a grin. 'It didn't seem relevant at the time.' He extended his hand. 'Help me up, please.'

Kohl and Nemiel hauled Askelon upright. The Redemptor glanced warily at the edge of the hole. 'Can the gun sense us up here?'

'To a limited extent, yes,' the Techmarine said. 'But activity overhead won't trigger a combat response. It's down there to guard the approaches to the building, and that's all.'