"What happened!?"
A strange, heavy silence descended upon the smoke-shrouded battlefield. Amidst the lingering spore clouds, there was only a deathly stillness.
An Imperial Guardsman clutched his Lucius-pattern lasgun with white-knuckled intensity, staring into the hazy, yellowish miasma, desperate to identify his next target. The trenches were a mosaic of ruin: fragmented human remains lay alongside the mangled carapaces of Hormagaunts that had been hacked apart by chainswords upon breaching the line. Near the mouth of a reinforced dugout, the carcass of a Tyranid Warrior lay slumped at an unnatural angle. In the troughs of the trenches, blood had pooled into dark, stagnant meres.
"Did we lose? Has the swarm moved past us?"
From another firing slit a few meters away, another soldier dared to lean out from his cramped position, peering into the gloom. Ten meters behind them, soldiers in the secondary support line were also looking about, their weapons held in a state of wary readiness. Men called out to one another, their voices rasping as they asked if anyone had seen anything.
There was only silence.
The frontline commanders had also taken note of the anomaly. Though the spore-choked fog had not yet dissipated, the guttural shrieks of the swarm had vanished. The perpetual, maddening drone of leathery wings in the sky had fallen mute. The Tyranid aerial organisms had simply disappeared.
The Imperial air elements had long since been expended. Countless Valkyries and Avenger Strike Fighters had taken flight only to never return. With the enemy's status unknown, it was deemed an unacceptable risk to use precious remaining Arvus Lighters for reconnaissance. As the Emperor's cheapest currency, human life was often valued less than a functioning machine; thus, dozens of scout squads across the sectors donned their respirators and advanced into the fog to uncover the truth.
Within the spore clouds, the dead of both sides were piled high, filling every trench. The craters left by detonated melta-mines had become basins for gore. The scouts moved with agonizing care, their limited vision heightening every instinct.
After navigating countless trenches and mountains of corpses, the shattered, towering bastions of the Hive finally loomed before them.
There were no xenos. Not a single living organism was to be found.
Emboldened, the scouts began the arduous climb over the mounds of chitinous dead clogging the breaches in the wall. When they finally crested the jagged ruins of the outer ramparts, the scene beyond was laid bare before their eyes.
The vast expanse outside the Hive walls had been divided into eight distinct sectors. The spore clouds had been forcibly dispelled. Upon each blackened stretch of earth stood a metallic structure of alien geometry. Legions of machines, gleaming with a cold, burnished silver, marched in and out of these hubs.
Surrounding this blackened ground were the incinerated remains of a Tyranid host beyond counting.
…
A Destroyer-class Heavy Automaton casually crushed the skull of a synapse creature in its power claw. Upon landing, the Hive Tyrant that had been leading the assault at the breach had been snuffed out by the automaton with effortless brutality. A Gravity Shredder had pinned the beast in place; though the Tyrant's bio-engineered resilience had kept it from being instantly pulverized by the sudden gravitational shift, it had been rendered immobile, unable to evade the massive power claws that stood nearly as tall as the beast itself. A single sweep of the pale-gold talons had rendered it into a pile of offal.
With the synapse link severed, the swarm began to fracture. Some beasts blundered mindlessly toward the Imperial lines, while others scattered in a primal panic. As the Iron Man legions deployed, they formed a net-like battle line that covered every vector of the xenos' advance.
The sounds of this new slaughter were swallowed by the thunder of the Imperial artillery in the rear. Densely packed beams of light acted like scythes through wheat, reaping the swarms that the Imperial defenders had viewed as inevitable doom. The Vengeance-class units unleashed their hybrid beams in interlacing patterns; with their turrets rotating slightly, entire swathes of Tyranid organisms collapsed as if their strings had been cut.
These xenos had not been biomorphically adapted to face the Iron Men. Against such mechanical precision, they were helpless. The Destroyer-class units rampaged across the field, their nine-meter power claws spread wide, reducing everything they touched to molecular dust. Aerial swarms attempted to dive-bomb the titans, only to be erased mid-air by the discharge of Energy Shockwave Weapons. Against such dense biological clusters, a single pulse sufficed for total eradication.
The melee-pattern Armored Wardens followed in the wake of the Destroyers, their power blades singing as they conducted a unilateral massacre. Within a few hours, Axion's mechanical army had driven the remnants of the swarm back into the contaminated oceans.
Oceanic combat presented a mixed variable for the mechanical legions. The advantage was that heat dissipation was no longer a concern. The disadvantage was the significant energy attenuation of beam weaponry underwater and the increased drag on locomotion. While Axion's databanks contained no mention of specific Tyranid aquatic sub-species, he made a cold, logistical decision regarding the environment.
Rather than engaging in the depths, he would simply evaporate the sea.
The Iron Men knew nothing of impatience. It was merely a matter of time.
From orbit, plasma batteries began a high-energy bombardment of the ocean. The mechanical army stood sentinel along the coastlines. Arcs of violent electricity surged through the water, electrocuting smaller Tyranid organisms instantly as the surface began to boil.
The Destroyer-class units lined the shore, their Heavy Destroyer Incinerators spewing pillars of white-hot ion-flame into the surf. Tyranid spores were incinerated on contact. The water itself began to undergo thermal dissociation, triggering secondary explosions that tore through the submerged xenos.
Massive white clouds of steam billowed into the atmosphere, only for Axion to detonate orbital plasma charges to blow the vapor out of the atmosphere or toward the planet's uninhabited poles. To balance the rapidly rising planetary temperature, Axion utilized his transports' gravity well systems to drop a massive ice-meteor onto the far side of the world.
After a brief period of "distilling" hundreds of millions of cubic meters of water, and physically purging the "residual impurities," the planet's mean temperature had risen by only thirty degrees.
Axion watched the planet, which was effectively experiencing a rain of boiling water, with total indifference. While the Imperial survivors on Aurelis were still trembling in fear of a new enemy, Axion was already recalling his legions and generating his combat theater report.
[In accordance with the request of Lord Regent Roboute Guilliman: Swarm eradicated. Combat operations concluded.]
With the broadcast of this wide-band signal, Axion's fleet broke orbit and vanished once more into the Immaterium.
The Imperial forces tentatively dispatched several Valkyries for reconnaissance. They returned with reports that the strange machines and metallic hubs had vanished, leaving behind at least a billion xenos corpses. Most baffling of all was that the planet's former ocean had been reduced to a vast, parched basin of vitrified glass.
Though this had not been a designated mission, Axion did not believe Lord Solar Leontus would reject the combat log. With his expanded forces and a single unified battlefield, the total expenditure had been a mere day of operations. To an Iron Man, time held no intrinsic value outside of a mission parameters. Machines do not age; steel can be replaced.