Under the "permission" of the Iron Tsar, the Imperial negotiators endured the exorbitant demands of the resistance with uncharacteristic patience. Although they did not immediately grant these requests, the diligent diplomats maintained the talks at a baseline where both sides believed they could continue "discussing" terms.
This attitude of Imperial forbearance made the defenders within the fortress even more confident in their eventual victory. They became convinced that the Imperium lacked the resolve to continue fighting and that they would soon celebrate a grand triumph over the invaders from the stars.
Consequently, the previously rigorous patrols atop the battlements became sporadic. Sentries began checking in merely to fulfill their quotas, no longer bothering with detailed scouting or vigilant observation.
Because of this, the construction detachments dispatched by Perturabo remained undetected. Taking advantage of the defenders' negligence, the Iron Warrior combat engineers utilized tunneling machines modified from industrial mining equipment. They began their excavation from a camp located a great distance from the fortress—a precaution taken to ensure the immense noise of the machinery would not alert the enemy.
From there, the Iron Warriors constructed a complex network of tunnels. These subterranean arteries snaked out from the Imperial camp, extending directly beneath the foundations of the bastion held by the resistance.
In the Iron Tsar's own design, these tunnels were not merely intended to house high explosives to be detonated at the start of the assault. Their more critical function was to transport elite detachments into position directly beneath the heart of the fortress. When the general offensive began, and while the defenders were reeling from the wall breaches and unprecedented fire saturation, these hidden warriors would burst from the earth to strike from within.
On the surface, these plans seemed to be tactics created by predecessors long ago, perhaps failing to reflect the singular genius of a Primarch.
To the Iron Tsar, however, this was irrelevant. So long as the objective was achieved with maximum efficiency, Perturabo saw no reason to discard proven methods in favor of "creating" unverified new ones for the sake of empty vanity.
While the Iron Tsar was not a man who cherished the lives of his sons for sentimental reasons, his precise calculations dictated that no Iron Warrior should waste their life needlessly. If a battle did not warrant a specific sacrifice, he would not squander his Legionaries. This restraint was partially a reaction to Rogal Dorn's stinging remarks, but more significantly, it was a product of the harsh environment of Kislev that had shaped Perturabo since childhood.
The chronic labor shortage on Kislev meant that historical Kislevite states preferred utilizing criminals for forced labor rather than executing them or leaving them to rot in cells. This background led Perturabo, the Tsar of Kislev, to view his sons as usable resources.
Since they were "resources," war became a business for Perturabo—one where losses could be quantified and measured. This perspective prevented him from throwing lives away recklessly; he demanded that every fallen Iron Warrior's death yield a tangible value.
To fulfill his vow of taking the fortress swiftly, Perturabo conducted meticulous preparations. In addition to the tunneling operations, he consolidated the artillery batteries of both the Seventh and Fourth Legions, preparing for a barrage of unprecedented scale.
Alongside this bombardment, the Iron Warriors and the Kislevite Streltsi were massed for a grand military assault. This would be the Fourth Legion's first military strike on this world, and it would be the last. Once it concluded, no resistor would dare stand against them.
After several days of intense engineering and preparation, the components of the Iron Tsar's plan were in place. The final battle was about to begin.
Accpanied by the thunderous roar of explosions emanating from the walls and the violent shelling from the Imperial camps, the defenders were jolted awake. The Imperium had struck without warning!
Before they could even condemn the Imperium for tearing up the negotiations and attacking without a declaration of war, the resistors scrambled to assemble their defenses.
But they quickly realized, to their despair, that the fortifications they once took pride in had been undermined. The explosives Perturabo had ordered placed beneath the foundations detonated, sending the walls—and the troops stationed upon them—skyward in an instant.
More catastrophically, Perturabo had concentrated every artillery piece within his reach to create overwhelming fire superiority the moment the siege resumed. The effect was exactly as he had calculated; under the crushing weight of the barrage, the defenders could not even lift their heads, let alone organize a coherent defense.
Taking advantage of the suppression, the Iron Warriors and the Kislevite Streltsi began their combined-arms assault. These highly trained Astartes and mortal soldiers were intimately familiar with the firing intervals of their own artillery. Moving in the gaps between the reloading cycles, they surged from their trenches and launched a massive offensive against the breached fortress.
Simultaneously, the elite units deployed through the tunnels long before the battle began burst from the ground at the center of the fortress. They struck the heart of the resistance just as the outer defenses crumbled.
Under the Iron Tsar's flawless calculations—trapped between superior enemy fire, collapsing fortifications, and an internal breakthrough—the defenders who had long withstood the Seventh Legion finally broke. After a significant portion of their forces was annihilated by the Iron Warriors and the Streltsi, they finally raised the white flag.
The ancient fortress that the Seventh Legion had failed to take fell within a few hours of the conflict resuming, just weeks after Perturabo took command of the line.
Naturally, the Iron Tsar could not help but feel smug. He looked forward to showing off his prowess before his golden-haired brother. However... he had underestimated Dorn's temperament and forgotten exactly why his brother had earned the nickname "The Stone."