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.

Scrabbling claws tore at the skiffs roof, gouging long tears in the thin metal. The skiff swayed from side to side, throwing more sparks as the pilot fought to compensate for the additional weight and drag of the attackers. Soldiers fired through the roof, killing the flying beasts in their dozens, but unable to dislodge them all.

Muscular taloned arms reached in and dragged a screaming soldier through a hole in the roof, his cries cut off as the rushing wind snatched away his breath. His comrades fought to pull him back, but another volley of the flesh-eating creatures slew the would-be rescuers in a hail of fire.

The skiff screamed around another bend in the icy canal, only to be faced by yet another flock of the winged monsters, swirling in an impenetrable cloud and blocking the skiffs path with their bodies. The pilot reacted instinctively, slamming on the air brakes and wrenching the controls to one side. Barbed brakes deployed from the skis, throwing the skiff into an uncontrollable skid.

The rear end of the skiff fishtailed, the passenger compartment slewing around until it was travelling sideways. Wider than the canal, its back end caught the edge of the mag-rails and flipped it over onto its side. At such high speeds, the impact ripped the passenger compartment open and tore the coupling to the engine with it, sending it spinning into the air to crash down onto the ice a hundred metres further down the canal where it exploded in a searing orange fireball.

Flames billowed skywards as the wreckage skidded along the canal for another six hundred metres, the heat from the flames melting the ice as it slid. Amid the carnage of the crash, a few pitiful survivors crawled from the wreckage, battered, bloodied and dazed.

Even before they had a chance to freeze to death, the winged gargoyles were upon them, biting and clawing at their helpless prey until there was no one left alive.

The first victims of the land war of Tarsis Ultra had been claimed.

From their perch high on the roof of their warehouse hideout, Snowdog and Silver watched the distant contrails of torpedoes as they climbed through the purple skies into the upper atmosphere.

The devotional holos, normally full of nameless preachers demanding prayers to the Emperor, had been displaying a non-stop procession of warnings against the dangers of contact with xeno species, Snowdog didn't know what was happening with the war, but was pretty certain that there must have been a screw-up somewhere along the line, because you didn't start firing ground based weapons except to prevent an imminent invasion.