He risked a glance over his shoulder to see why no one was shooting the damn thing down. As he saw the nearest Hydra he realised why.

Its frontal section was a molten, twisted mass, thick armour plating liquefied by corrosive viruses and acids. Gory slime oozed from the vehicle's interior, the disintegrating flesh of its crew steaming in the cold air. But Konarski saw the Hydra's gun section was still intact.

He dropped his rifle and sprinted towards the quad-barrelled gun. He had to get it firing again. Huge, shrieking creatures with scything arms and horrific organic weapons poured over the trenches, tearing his men apart. Swarms of smaller creatures leapt and killed around them.

Desperate hand-to-hand combat raged as troopers vainly attempted to stem the alien tide. Giant, fleshy monstrosities disgorged hordes of clawed monsters that he recognised as genestealers. Everywhere, they were being overrun.

Konarski crouched low and held his gloved hand across his nose and mouth as the stench of melted human flesh assailed him. He scrambled across the stinking remains of the crew, sliding up into the gunner's compartment.

'Yes!' he shouted as he saw that the guns were still powered up and fully loaded. Gripping the firing handles, he slewed the four-barrelled turret around to face the giant flapping monster. Konarski punched the firing studs and a four-metre tongue of flame roared from the muzzles to strafe the sky with fiery explosions. The gun rocked with powerful recoil, pumping out hundreds of shells every few seconds. Konarski screamed as he fired, the horror of the last few days washing from his body in a storm of adrenaline.

Through the vision blocks he saw the flying beast torn apart as the close range blasts ripped through its bony armour plates to detonate within its vital organs. Screeching, it tumbled from the sky, rolling in a flurry of snow and alien blood to crush the broods it carried with its bulk. Explosions of coloured fumes erupted from its belly, noxious clouds of alien toxins blanketing the ground and green tendrils spilling into the trenches.

Working the gun left and right, he shredded every alien he could see, keeping the firing studs depressed long after the ammunition had run out.

Colonel Rabelaq watched through the viewing bay of the Capitol Imperialis and immediately saw that the Krieg rearguard was sure to be annihilated unless they were reinforced. Cries for help and desperate pleas for fire missions clogged the vox-circuits. The scale of the disaster staggered him.

The elements ambushed on the road to the city were holding, and in many places driving the tyranids back. Given time, Rabelaq guessed they could probably fight their way behind the walls. But time was the one thing they did not have.

The soldiers of Krieg could not hope to hold the tyranid advance long enough.

There was only one thing to do.

He marched to the centre of his command bridge and buttoned his frock coat, pulling the collar straight and brushing a piece of lint from his epaulettes.

'General advance, ready main gun,' he ordered.

'Sir?' queried his adjutant.

'You heard me, damn you! General advance, I'll not leave those brave lads to fight and die on their own. That's not the Logres way. Now do as I order!'

'Aye, aye, sir,' nodded the man, hurrying to obey.

Colonel Octavius Rabelaq came to attention as he felt the rumbling vibrations of the gigantic tracks and the Capitol Imperialis began its ponderous advance.

The ground shook, the charge of hundreds of alien monsters dislodging snow, ice and timber from the walls of the trenches. Konarski grabbed whatever men he could find through the stinking clouds of alien fumes, hauling them back towards the city wall. They had done as much as they could, and it was time to get his men to safety.

Huge vibrations rumbled through the ground, and briefly he wondered if they were in the grip of an earthquake. A screeching roar behind him echoed with alien hunger and he turned to raise his lasgun in a final show of defiance.

Suddenly the earth heaved and a thunderous string of explosions filled the world with noise. Bright light flared behind him and the crack of displaced air threatened to deafen him. He felt himself flying through the air as massive tremors split the ground before him. He hit hard and rolled, swallowing snow as stars burst before his eyes.

Flames leaped before him and he pushed himself dizzily to his knees.

What the hell had just happened?

Then the smoke parted and he saw a towering cliff of steel rising before him. Grinding forward on lumbering tracks that crushed the earth, it split the very bedrock with its mass, throwing up tank-sized chunks of ice and rock. The blessed sight of the aquila was emblazoned on the soaring leviathan, just below the gigantic, smoking barrel of the Behemoth cannon mounted on the Capitol Imperialis. Konarski laughed as the mammoth war-machine rumbled past him, his cry of exultation snatched away as its cannon fired again, the concussive force hurling him through the air once more.

The landing knocked the breath out of his body, but fuelled by adrenalin, he quickly staggered to his feet and lurched off in the direction of the city.

Colonel Rabelaq had bought them time and he wasn't about to waste it.

Colonel Stagler kept the compress bandage tight against his stomach, dizzy from blood loss, but unwilling to accept medical attention until he knew the fate of his men. Even from his vantage point on a snow-capped gun tower atop the main wall, billowing clouds of smoke and fumes obscured his view of the trenches. He could get nothing from the vox-caster, simply screams and alien howls. His men were probably lost, but they had died in the Krieg manner: fighting hard and dying well.

The fool Rabelaq had surprised him, pushing his precious mobile command post into the alien mass. He'd bought the men fighting the ambushing aliens enough time to break free of the noose and escape to the transient safety of the city. Entire broods of aliens had circumvented the walls, dropping from the high cliffs and into the depths of the city, but he couldn't worry about them right now.

The Capitol Imperialis fired again and more snow tumbled from the highest peaks of the mountains. Hundreds of aliens swarmed up the flanks of the mighty vehicle, many more slamming their bulk into its tracks. Electrical discharges erupted around its hull and bright explosions surrounded it. Its close-in defences stripped away whole swathes of attacking aliens, but could not cope with the sheer volume of attackers.

Stagler snapped his fingers in the direction of his vox-operator.

'Get me Colonel Rabelaq,' he ordered as he saw a sight that would stay with him until his dying day.

'Why are we slowing, damn you?' demanded Colonel Rabelaq.

'Sir, the track units are jammed. We can't move,' came the reply.

The commander of the Logres regiment rushed to the surveyor station, where dozens of small pict-slates displayed images from the external viewers. Flickering scenes of carnage filled every one, thousands of tyranid brood creatures swarming around the Capitol Imperialis. Hundreds of short-range bolters fired a continuous stream of explosive shells into the alien horde, but could not stop them all.

He felt the recoil-dampened vibration of the main gun and even through the thick hull of his command vehicle, he could hear the shrieks of the deadly aliens as they fought to get at the humans inside his armoured behemoth.

Hundreds, perhaps thousands of aliens had thrown themselves into the mighty tracks of the Capitol Imperialis to prevent it from escaping, and the scale of such unthinking devotion terrified Rabelaq to the soles of his boots. Not even the ruthlessly driven Macharius or the charismatic Slaydo had inspired such obedience from their warriors.