In many cases, this undoubtedly saved their lives.
From the ridges of the northern mountain slopes, hundreds of tyranid organisms poured down the treacherous, rocky slopes to fall upon the strung-out Imperial forces.
Soon, fierce battles were raging before the city walls as a tide of alien killers, having traversed the supposedly impassable mountains, fell upon the unsuspecting Guardsmen.
Alien and human blood flowed in rivers as the two forces clashed.
But there was worse to come.
'Oh, sweet Emperor, no…' moaned Colonel Rabelaq as the images on the holo-map suddenly leapt forwards. Fresh enemy icons appeared on the northern mountains and he realised that Captain Ventris had been right to doubt Fabricator Montante's word regarding their impassability. Fear settled in his belly and the blood drained from his face. The tyranids had fooled them all. The calculus-logi of the Capitol Imperialis had projected the speed of the advancing swarms and assumed that they were moving at optimum speed. Naively, he had fallen to thinking the same thing, but as he watched the icons of the three swarms closing rapidly with Erebus, he realised that he had fatally underestimated the cunning of these aliens.
He rushed towards the vox-station, and grabbed the carved nalwood handset from the console.
'All Krieg units, be advised that the tyranids will be on you imminently! I repeat, the tyranids will be attacking your position within minutes! Get out of there now!'
'What the hell are you talking about?' snapped Lieutenant Konarski, grabbing the headphones from the vox-operator and jamming them to his ear. His eyes widened as he heard the panicked voice of Colonel Rabelaq screaming for them to evacuate the trenches.
He tossed back the handset and ran to the trench periscope, pressing his face to the viewing plate. Biting back a curse he swung the scope from left to right and felt a cold band of iron close around his chest as he saw a tide of alien monsters hurtling towards their position.
'Shit,' said Konarski and unslung his lasgun from his shoulder.
He ran along the trench, shouting at his men to stand to.
'Sir!' called his vox-operator. 'We're not evacuating?'
Casting his gaze along the line of the trench and seeing other Krieg officers pushing their men onto the trench's firing step, he said, 'No, son, we're not.'
'But Colonel Rabelaq's orders…'
'Damn Rabelaq!' snapped Konarski. 'We're the Death Korp of Krieg, son. Did you think that was just a pretty name? We never retreat. We fight and we die, that's the Krieg way.'
As terrifying as the first attacks on the trenches had been, they were but a shadow of this assault. A massive, multi-limbed beast stamped forwards, smashing giant craters in the ice as it charged. Steaming jets of scalding acids sprayed from grotesque organic tubes slung beneath its massive jaws, dissolving snow, ice and flesh in smoking conflagrations.
Hundreds of spines fired with monstrous muscular contractions hammered the trenches, punching through metres of snow to skewer both men and tanks.
A boiling tide of creatures swarmed around the legs of the gargantuan beast. Chitin-clad organisms with bony prows and curled forelimbs hurled fleshy pods which burst in lethal sprays of razor-sharp bone and bio acids. Slow moving, each creature excreted another organic missile as it slithered across the ice.
Similarly bulky creatures, with fused, bony forelimbs that resembled long, organic cannons spat crackling chitin shells that hammered the retreating tanks with sprays of corrosive viruses and acids. Crackling electric energy leapt from the giant claws of thick, serpentine creatures that hurtled across the ice, their rasping armoured hides throwing up clouds of ice crystals in their wake.
But leading the charge, faster even than the multitude of ravening organisms that made up the bulk of the tyranid swarm, was a clutch of enormous creatures that smashed their way forwards on gigantic claws that dragged their bloated bodies across the ice with terrifying rapidity. Brood nests pulsed with a grotesque peristaltic motion between the bony plates of their hides and rippling muscle contractions hurled razor-edged spines towards the trenches.
A dark cloud of gargoyles massed above the attacking aliens, a massive black brood-mother moving amongst them, its monstrous wings flapping ponderously as it descended towards the men of Krieg.
Lieutenant Konarski retched as he pushed the dissolving remains of his vox-operator from his legs to fall into a pool of smoking acids that melted its way through the trench's duck-boards. He tried to stand, but the acrid stink of seared flesh doubled him in up with a fierce coughing fit. Blood and smoke filled the trench as tyranid missiles burst around their shattered defences. Here and there shots were returned, but it was a drop in the ocean compared to the fire they were receiving.
Finally overcoming his nausea, he shouted, 'For Krieg!' and fired over the lip of the trench. A dark shadow blotted out the light from the sun and Konarski looked up in time to see a gigantic monstrosity with wings tens of metres wide swooping low towards the trenches. Scores of smaller beasts clung to its belly and a swirling fire built between its jaws.
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.