For the forces of the Imperium, the eruption of this battle was impossible to process.
Upon identifying their targets, the Iron Men moved with terrifying immediacy. Fire suppression, wide-area bombardment, synchronized orbital drops, and mass-scale deployment, processes that should have occupied hours, if not days, of standard warfare were compressed into less than ten minutes.
In the distance, the surface of Medrengard roiled with heat intensities no biological entity could survive. The Iron Men's legions, however, felt nothing. Their gargantuan weapons struck the fortress bastions with force beyond comprehension; shields were shredded, armor was liquefied, and rivers of dark-red molten slag poured into the compartments, burning even the Astartes alive within their own walls.
The Iron Men delivered their forces to the surface with impossible efficiency, beginning an indiscriminate assault on everything the planet held. The firepower of the Iron Warriors became a joke in their presence; the transhuman might of the Astartes held no advantage against the machines, and their particle oscillation weaponry proved equal to, if not superior to, any power weapon.
The White Scars and Space Wolves, who had prepared to join the fray, stood in stunned silence as they observed the "slaughter" through the panoramic observation systems of the Void Sword. Even Jaghatai Khan, upon witnessing the carnage, could not help but clench his fists. He had brought eight thousand sons, warriors who had followed him since the days of the Great Crusade, but the Iron Warriors were equally formidable.
During the Horus Heresy, Perturabo had taken a nearly intact Legion with him into rebellion. Beyond the Warsmiths who roamed the Imperium's borders, raiding worlds and establishing recruitment nodes, the vast majority of the Iron Warriors were entrenched on Medrengard. The planet was not only home to Perturabo himself, but also to nearly a hundred thousand Iron Warriors, a staggering number of Chaos sorcerers, and billions of Warp-tainted mortal slaves. Added to this were the countless war machines built by the Iron Warriors: massive armored clusters, impenetrable bastions, and a host of perilous mechanical monstrosities.
Yet, before the Iron Men, all of this appeared fragile.
A single Sentry-Trooper could fight an Astartes to a standstill. But when hundreds of millions of such "Astartes-level" machines were dropped onto the surface at once, the reality became nightmarish. The Iron Warriors struggled like candles in a gale, unable to resist. Astartes fell in swathes like mortals; armored columns were crushed by towering mechanical units; rays of energy and detonations tore the sky asunder. Colossal machines tore through tank battalions with roaring power claws. Destroyer-class Heavy Automata, moving in clusters of thirty to forty, surged across the ground, each one significantly larger than a Warhound Titan, yet possessing several times the agility. There were hundreds of such "Titan" clusters on the field.
Before this moment, no one had dared imagine what thousands of Titans fighting in concert would look like. Now, the Imperial forces beheld that horrific spectacle.
All resistance was futile. Remnant bastions were torn open; the Knight-suits of Chaos Knight Households were overturned like children's toys; the Warhound and Warlord Titans of the Chaos Titan Legions were shattered with ease. Tanks were trampled flat, shells ricocheted harmlessly, and beams of energy rippled across void shields, unable to leave a mark. The speed of the machine-entities was so great that even the heaviest weapons struggled to track them. The Warp-tainted slaves were harvested in harvests of blood, the slaughter total and absolute.
Grotesque mutants, wandering the hills and ravines, shrieked without reason as they charged the machine-entities, only to be reduced to piles of shredded meat.
For the first time, they witnessed the Iron Men utilizing forbidden, taboo weaponry. A directional radiation cannon swept across the landscape, enshrouding the entire earth; flesh dissolved from bone, and skeletons crumbled into brittle, lifeless dust. A single volley turned hundreds of thousands into ash, transforming the terrain into a boiling pool of blood.
Arc-weaponry mounted on the machines discharged in cascading, leaping bolts. These arcs pierced armor, not only paralyzing the Iron Warriors' weaponry and causing chain-explosions in their vehicles, but cooking the biological matter within the power armor into charred ruin. The Peltast Sniper Automata deployed nanite-devourer munitions at long range, and these terrifying grey mists erupted within the throngs. Vehicles, wargear, Astartes, and mortal slaves were consumed indiscriminately, forming a grey-black tide that swept over everything.
Even the Dreadnoughts of the Iron Warriors, hidden within the underground tunnels beneath the defensive lines, fared no better. Their heavy bolters, meltas, and multi-lasers were like pinpricks against the Iron Men. Their power claws and fists were effective, but woefully inefficient. Worse yet, the number of mechanical entities outnumbered their own slave-mechanics by tens of times.
Having studied Imperial technology, the Iron Men were intimately familiar with the flaws of these Dreadnoughts and other engines of war. The TR-09 Morlanad assassin robots that flickered into existence on the battlefield repeatedly inserted their blades into the sarcophagi of the Dreadnoughts at speeds exceeding reaction limits, shredding the remaining organic matter and circuitry within.
Even Jaghatai had to concede that the Iron Men's assessment of their role had been without error: in this theater of war, they were little more than baggage.
Soon, they noticed something peculiar about a towering fortress atop a mountain range, not for its scale, but because the giant Titans of the Iron Men were climbing it. Several Punisher Titans swung their power claws, dismantling the entire fortress bit by bit as if meticulously searching for something. Channels deep within the mountain were exposed; secret chambers were laid bare. A myriad of mechanical units of all sizes scurried through the fortress, chasing and scouring every hidden corner, which were forced open one by one. The defensive turrets on the mountainside fired frantically, but they would fall silent after only minutes of engagement.
Deep within the fortress, Perturabo was sprinting. Thanks to his past experience, he had left behind a vast network of observation devices and hidden windows, allowing him a clear view of the situation outside. As the Primarch of the Iron Warriors, he had never been so humiliated. He had considered his bastions the embodiment of the unconquerable; defenses and firepower so formidable that even Dorn and Ferrus would have been forced to concede defeat.
But now, the Iron Men's assault told Perturabo that his pride was a joke in their presence. Their horrifying firepower obliterated his fortifications and the warriors within them with contemptuous ease. The Iron Men, specialized in slaughter, were unleashing their true nature. And even the daemonic entities of the Warp, drawn by the mass-scale death, stood no chance against them. The psychic arcs of the Luminous units and the terrifying beams of the ectoplasmic dissociation cannons tore the daemons apart by force.