The battles prelude began in mid-M41 when the now-ancient Iron Father Kristos (still mired in controversy from the Kristosian Conclave) vowed to hunt down the Emperor's Children of the Sapphire King after a report of their presence had come from the Gaudinia System. The Iron Hands fleet that made way for the System was huge, having eight hundred Battle-Brothers under its control. The Marines found that all eight worlds of the system were overrun by Mutants and heretics, but that the Emperor's Children were only present on Gaudinia Prime. The Iron Hands thus made this their primary objective, with all of Clan Raukaan and Clan Sorrgol (nearly 300 Marines) directed at the planet.
The Iron Hands blasted through fleets of Imperial Navy Frigates that had become corrupted by the Sapphire King. However as the Iron Hands arrived over Gaudinia Prime they found no lifeforms on the planet, neither Imperial or Chaos Space Marine. Instead, all life readings appeared to be concentrated into one factory complex in the planet's southern hemisphere. Kristos refused to listen to pleas for caution from Iron Fathers Stronos and Verrox, launching an immediate attack on this target. Bike and Land Speeder formations from Clan Morragul were landed in the Manufactorum City but found no sign of any foe. Still, the Iron Hands’ auspexes were reading life-signs from all around. As they worked their way toward the central processing hub they began to hear sounds of industry. Bolters swung up and squads moved into battle formations as the Space Marines approached the vast iron dome of the hub, listening to the frantic cacophony that rang from within. The hub stood almost two thousand feet high at its crest, ten miles across at its base, and appeared to be sealed tight as though against attack or unnaturally severe weather. From within came a maddened din of machinery interspersed with hissing groans and wheezing screams that caused even the Iron Hands to look askance at one another. Borne on the wind was a stench like burning flesh mixed with some kind of bilious sweetness as though gallons of perfume had been spilled into rotted faeces. Impassive, Kristos ordered entrances to be made in the dome’s walls. He and the other Iron Fathers would lead the warriors of Clan Raukaan to discover what manner of devilry lurked within.
A probing attack by Ironclad Dreadnoughts and Assault Centurions revealed that the workers and machines within the dome had become one. Billions of Imperial citizens had been crammed into a ten mile space, their flesh bonded with steel. Human torsos, skin seared and impossibly bloated, jutted from boiler-stacks. They shrieked endlessly as steaming blood vented from their eyes. Here, cogs of bone and raw, bloodied nerves rotated at breakneck speed. There, daemonic weapons were thrust along fleshy conveyor belts by the peristaltic motion of a billion disembodied tongues. Worst of all was the din, a discordant industrial thunder of jarring voices that tore at the air until the Iron Hands were forced to dampen their audio-receptors. Nonetheless Clan Raukaan advanced to the heart of this structure, but Kristos found himself overwhelmed by what was taking place around him. Muttering Can you not see it, Graevaar, Do you not hear its song? The perfect utility... the efficiency... the strength..., Kristos plunged a nest of his mechadendrites into the fleshy receptors of the machines around him. Soon enough it became apparent that Kristos had given himself to the forces of Chaos, and was transformed into a hideous bulging machine-spawn of flesh and metal.
The corruption did not end at Kristos however, those loyal to him (known as Kristosians) soon became overcome by the "perfection" of these flesh-engines and succumbed to the same malignancy as the Iron Father. Soon enough Daemons began to emerge from these fresh hosts, assaulting whatever loyal Iron Hands remained. With them finally came the Emperor's Children, their trap complete. Madness engulfed the dome as a hellish battle between the remaining Iron Hands, flesh-machine-spawns, and the Emperor's Children erupted. The Sapphire King took this opportunity to finally reveal himself, and Kardan Stronos was forced to rally his battle-brothers and gun down many of their comrades who had fallen to the taint of Slaanesh. Stronos began a coordinated retreat from the dome, fighting furiously all the way.
As the last surviving Iron Hand exited the dome's breach, the tanks of Clan Raukaan struck and exterminated Daemons in droves. Still the hordes of the Sapphire King came, and the Daemon himself was seen with a retinue of Noise Marines. The Iron Hands now found themselves being slowly overwhelmed by these foul hordes, and Epistolary Lydriik tried to save the situation by charging into the Sapphire King's cadre. Inspired by this act of heroics, many Battle-Brothers joined the Epistolary. At the height of the melee, Lydriik managed to use the Mindforge Stave to behead the Sapphire King. With their lord destroyed, the Daemon hordes began to fade away while the Emperor's Children fought on with a mad glee. However these isolated Chaos Space Marines were swiftly destroyed by Clan Raukaan.
Horrified by what they experienced, the Iron Hands burned Gaudinia Prime from orbit. It was eventually discovered that a third of the Iron Council had been killed or corrupted during the Gaudinian Heresy