'I have a task for you, Brother-Redemptor Nemiel,' the primarch said. Beside him, Brother Titus stood sentinel beside the assembly building's open doors. A plasma bolt had fused the barrels of his assault cannon, but his deadly power fist still functioned.

'What are your orders, my lord?' Nemiel replied calmly.

'It's absolutely vital that the siege guns do not fall into Horus's hands,' Jonson replied. 'Do you agree?'

Nemiel nodded. 'Of course, my lord.'

'Then we must take steps to ensure that they are destroyed in the event that the Sons of Horus break through,' the primarch said. 'I want you to find Techmarine Askelon and instruct him to prepare a demolition device that will destroy the assembly building and everything within it. According to him, the siege guns' ammunition sections are fully-loaded. If he can rig the shells to detonate it should devastate everything within a five kilometre radius.'

The Redemptor nodded sombrely. The order wasn't unexpected. Once he'd heard a full tally of their losses he knew that their odds of victory were growing slimmer by the moment.

'I'll see to it at once,' he said.

He left the primarch and hurried into the assembly building. On the way he caught sight of Brother-Sergeant Kohl and the rest of the squad taking their place with the rest of their brothers at the inner line. For a moment he and the sergeant locked eyes, and Kohl seemed to understand what the grim look on the Redemptor's face signified. Nemiel gave the veteran a knowing nod, and the sergeant saluted him in return.

There were close to a hundred seriously wounded Astartes laid up inside the assembly building, their conditions monitored by the ground force's Apothecaries. Nemiel searched among the unconscious or comatose figures, looking for Askelon and frowning worriedly when he could find him.

'Up here,' echoed a familiar voice. Nemiel looked up to find the Techmarine standing atop the dorsal hull of the lead siege gun. Askelon pointed to the rear of the huge vehicle. 'There are ladder rungs back at the ammo section.'

Nemiel hurried back to the rear section of the war machine and scrambled up onto the dorsal hull. The armoured deck stretched for a hundred metres from one end to the other, nearly as long as an Imperator Titan was tall. He jogged down the length of the huge machine, joining Askelon by the open hatch where they'd watched Archoi's technicians work just a few hours before.

'What in Terra's name are you doing up here?' Nemiel asked. 'Brother-Apothecary Gideon said you should be resting. Your internal organs and nervous system were badly damaged when you tapped those power conduits.'

Askelon waved such concerns away. 'I'm not doing us any good sitting down there on the permacrete,' he said hoarsely. Burn sealant had been spread across his burned face, giving the charred skin a synthetic sheen. 'So I thought I'd climb up here and see if I can get this monster running.' Nemiel's eyes widened. 'Is that possible?' Askelon sighed. 'Well, in theory, yes. The engine is functional, the weapons fully loaded, and the void shields - all four of them - check out as ready for activation. The problem is that there aren't any manual controls!'

The Redemptor frowned. 'That doesn't make any sense. Even a Titan has a crew to assist its Princeps.'

Askelon nodded. 'And these vehicles were built with supplementary crew stations - but Archoi's tech-adepts took out all the controls and welded the hatches shut!' The Techmarine shook his head. 'It doesn't make any sense. I can't imagine what Horus thought he could use to operate these machines - the systems aren't quite as complex as a Titan, but they're close.' He spread his arms and gave a frustrated sigh. 'So here we are with the firepower of an army in our hands, and no way to use it.'

The Redemptor scowled down at the open cockpit. A thought niggled at him. 'Is there any way to rig a basic set of controls to the machine - even just to operate one of its void shields?'

Askelon shook his head. 'Actually, operating the void shield is one of the most complex operations to manage - just ask any Titan moderati. Rigging an effective set of controls would take hours, possibly days.' He shook his head. 'Unless you have a spare MIU sitting around, there's nothing we can do.'

Nemiel glanced up at the Techmarine, his eyes widening. Askelon frowned. 'What's the matter?' he asked.

'We do have an MIU,' Nemiel said. 'It's been right under our noses all along.'

The enemy launched their eighth and final attack an hour and a half later.

Lion El'Jonson listened to the approaching sound of engines and readied his sword. 'Here they come,' he said to Nemiel. Around them, the surviving Astartes checked their weapons. At the Redemptor's urging, they had extended the perimeter of the inner line outwards another two hundred and fifty metres, stretching their coverage almost to the breaking point.

'Askelon is working as quickly as he can, my lord,' he said to the primarch. 'We have to buy as much time for him as we can.'

'This is a terrible risk we're taking,' Jonson replied. Amid the mounting tension, the primarch managed a faint smile. 'If we fail to hold them back and somehow we aren't both shot to pieces in the process, I'm going to hold you personally responsible for this.'

Nemiel nodded. 'Duly noted, my lord,' he said in a deadpan voice that would have made Brother-Sergeant Kohl proud.

The enemy vehicles advanced from three sides, nosing their way through the maze of close-set buildings and closing in on the assembly building. By careful planning or sheer, diabolical luck, most of the enemy vehicles emerged from cover at the same time. Nemiel counted ten Rhino APCs and, directly ahead, a patched-up battle tank. A square metal plate had been bolted over the crater blasted in its glacis by a lascannon bolt, and the rebels' technicians had jury-rigged enough of its wrecked controls to get it back into action. The tank shuddered to a halt as the rest of the APCs surged forward. Its turret tracked fractionally to the left and the battle cannon fired.

The heavy shell howled through the air towards the Dark Angels and struck Brother Titus's armoured torso. The Dreadnought vanished in a thunderous blast, hurling bits of its arms and chest high into the air. Shrapnel rained down on the defenders, the metal fragments pinging off their armour.

Jonson straightened in the wake of the blast, his expression tense. Twenty metres away, the Rhinos came to an abrupt halt. Assault ramps dropped to the ground as ten squads of pale-armoured Astartes disembarked and took shelter behind the cover of their vehicles. Farther back, the tank traversed its turret to the left, taking aim on a Dark Angels squad.

'This won't work,' the primarch snarled. 'That tank will sit back there and shoot us to pieces, and then the Sons of Horus will sweep in and mop up the survivors.' He drew the Lion Sword and held it aloft. Sunlight shone on its razor edge. 'Forward, brothers!' he cried. 'For honour and glory! For Terra! For the Emperor! Forward!'

All sixty Dark Angels rose to their feet in a single, fluid motion and advanced towards the Sons of Horus, a thin line of black against a waiting phalanx of white. The battle cannon boomed again, but the gunner failed to adjust for the sudden enemy advance, and the shell blasted a gout of dirt and permacrete into the air behind the Astartes. The rebel warriors rose from cover and opened fire as well. Plasma bolts and shells stabbed out at the advancing Imperials, and the Dark Angels returned fire. The two formations drew inexorably together. Nemiel clutched his crozius tightly and prepared for one final battle.

A tremor rippled through the ground beneath their feet - very faint at first, but growing in strength with each passing moment. Nemiel felt it through the soles of his boots and turned to Jonson, who had felt it, too. A throaty roar filled the air behind them, swelling outwards in a solid wall of sound as one of Horus's mighty siege machines rumbled slowly onto the battlefield.