In minutes, the newly risen sun was obscured by the sheer mass of spores falling from the heavens. Terrified yrenbacks ran backwards and forwards across the mountain sides, leaping through the deep snow in their blind panic. Churning motion erupted from the steaming spores that had landed in their midst, flashing claws and alien screeches as the creatures within them emerged and sought something to kill. Driven into a frenzied killing fury by the hive mind and bio-engineered, super-adrenal chemicals, the first wave of tyranid invaders hacked entire herds of the grazing animals to bloody ruin before collapsing and dying, spent by the fury of their assault and their inability to survive the freezing temperatures.
Thousands of tyranid organisms in the first wave perished as the numbing cold of Tarsis Ultra froze them within minutes of their arrival. After burning virtually all their bodily energies in their initial surge of violence, and without reserves of fat, none could survive more than a few minutes before perishing.
But none of this mattered, for as each creature died and the hive mind became aware of the local conditions on the prey planet, it simply adjusted the biological physiology of its warrior organisms, enabling them to produce more insulating tissue and energy reserves that would allow them to survive for longer periods.
Amid the loamy earth of the lower forests, the thick, biological rain soaked into the tree canopy and saturated the earth with its bacteria-laden substance. Microbes containing the genetic blueprint of tyranid fauna spread rapidly through the ground, assessing and digesting the chemical content of the soil before turning that energy into horrifyingly fast growth spurts.
Multicoloured fronds ripped their way through the silver bark of the trees and twisting vines and creepers surged from the moistened ground. Again, the cold of Tarsis Ultra dramatically shortened the plants' lifespan, but as each leaf and creeper died, it vomited a host of fresh spores into the atmosphere and the cycle began again.
As each generation of plant went through its brief life cycle, the chemical reactions fermenting in the ground began raising the temperature of the surrounding air. Streamers of heated air drifted from the ground, warming the burgeoning plant life until the rate of growth was rising exponentially. Jagged spore chimneys of thick, vegetable matter broke through the hot earth and pushed skyward, their root structure burrowing through the permafrost to the nutrient-rich soil below. Hot steam and exhaust gasses from the biological conflagration below belched from the chimneys, sending yet more spores high into the atmosphere to be spread by the prevailing winds. As the atmosphere heated even more, strong updrafts of warm air rose, meeting the cold air descending from the mountaintops to create freak weather patterns that spread the contamination of the tyranid organisms even further.
The invasion of Tarsis Ultra had begun.
Despite the inability of the Imperial fleet to hold back the tyranid invaders, Tarsis Ultra was not without defences of her own. Ground-based batteries of defence lasers fired skyward and hundreds of orbital torpedoes roared into the upper atmosphere on blazing tail plumes.
The defence lasers slashed through the sky, but the rapidly mutating content of the air had one more adaptive surprise for the defenders of Tarsis Ultra.
One of the greatest problems for ground-based laser weapons was the reduction in power they suffered over long distances, called ''thermal blooming''. As a laser beam travels through the air, small quantities of its energy are lost to the surrounding atmosphere as heat, which causes disturbances in the air and disrupts the optical path of the beam. Not only does this impair accuracy, but it spreads the beam wider, thus weakening the energy delivered to its target. For the colossal energies produced by the defence lasers, this was not normally a problem, but each beam was passing through dozens of rapidly fluctuating temperature patches in the air, causing them to impact with greatly reduced power.
Many of the smaller organisms suffered at the hands of the defence lasers, but the majority of the tyranid creatures had little to fear from them.
But torpedoes have no such barriers to performance and these weapons reaped a fearful tally amongst the gathering predators. Hundreds of torpedoes exploded amongst the bloated spore ships of the tyranid fleet, destroying some and fatally wounding others. Scores of alien creatures perished and fell through the atmosphere as bright, fleshy meteors, haemorrhaging their lifeblood like comets' tails.
The skies above Tarsis Ultra were what Imperial strategos referred to as ''target-heavy'' and every torpedo found its mark in a tyranid creature. Within two hours, over five hundred confirmed kills had been reported by the silo commanders, along with desperate requests for additional ordnance to fire. Faced with so many targets, each silo exhausted its supply of weapons after another hour of firing.
Against any conventional invaders, the defences of Tarsis Ultra would have caused utter devastation and crippled any attempt to invade.
But the tyranids were far from conventional invaders.
From the air, the hydro-skiff resembled a speeding silver bullet as it roared along the frozen surface of the hydroway. Its passenger compartments were laden with soldiers of the Logres regiment making their way back to Erebus, its speed approaching two hundred kilometres per hour as its giant, prop-driven engines hurled it along the frozen canal surface.
A mist of ice crystals billowed in its wake as the hydro-skis angled to take the skiff around a bend in the canal, rounding a series of low hills capped with a thatch of evergreen firs. Sparks flew as the offside ski grazed the mag-rails on the side of the canal, the pilot having taken the turn a little too fast for comfort. But concerns of safety were now outweighed by the need for speed. They had seen the heavens criss-crossed by bright streaks of lasfire, and the pale blue of the sky to the west was laced with cloudy pillars from firing torpedo silos. No one needed to tell the men of the Logres regiment what was happening, and that it was time to head for the safety of Erebus.
Unnatural twilight was falling as tyranid spores filled the sky above, long shadows cast by chittering black clouds that spun and swooped like flying oil slicks. Soldiers peered nervously through the steamed windows at the gathering darkness, willing the skiffs pilot to coax his machine to yet faster speeds.
A pair of the black clouds dropped through the air, looping downwards to fly parallel to the skiff, a third descending in a lazy spiral ahead of it. Officers watching through the roof periscopes shouted at their men to stand to, chivvying them to the windows and bellowing orders to fire at will.
Blasts of freezing air filled the skiff as windows were forced open and barrels of lasguns pushed through. Lasbolts snapped upwards, punching into the black flocks that pursued the skiff. Occasionally, a twisted shape would tumble to the snow, but such precious victories were few, and despite the terrific speed of the skiff, the flocks drew nearer still.
Cries of fear echoed along the length of the passenger compartments as the flocks began to overtake the skiff and the soldiers had their first glimpse of the enemy. Grotesque, membrane-winged creatures with leering, fang-filled maws and clawed limbs surrounded them. Lasbolts tore amongst the aliens, but for every one that was downed, a "hundred more remained. They swooped over and around the skiff, spitting black gouts from weapon orifices that peppered its metal skin like handfuls of thrown stones. Glass smashed and men screamed as the aliens' fire struck them, their armour cracking and dissolving under the impacts.
Medics ran to the wounded, peeling off bloody flak vests and applying pressure to the ragged holes in the soldiers' bodies, then recoiled in horror as they saw clutches of writhing, beetle-like creatures boring deep into the men's flesh.