Istvaan III Atrocity



"Let this fire spread across planets and consume stars. Let all become the pyre from which the new age will be born."

- Erebus, First Chaplain of the Word Bearers, Words of Offering at the bombardment of Istvaan III

The Istvaan III Atrocity, also known as the Battle of Istvaan III to Imperial scholars, was the treacherous military campaign conducted by the Chaos-corrupted Warmaster Horus on the rebellious Civilised World of Istvaan III that served both to amass those forces loyal to him without suspicion, and also cull those elements within the ranks of the Traitor Legions whose loyalties he suspected as remaining true to the Emperor of Mankind. The destruction of Istvaan III by virus-bombing and the subsequent internecine battle that followed between the Sons of Horus, Death Guard, Emperor's Children and World Eaters Traitor Legions and their own Loyalist elements was the first overt sign of Horus' turn to Chaos, and the first battle of the Horus Heresy in the early 31st Millennium.

Istvaan System Compliance
An ancient Terran proverb once said that all wars, no matter how bloody or great, begin with but a single shot fired. For the great cataclysm known as the Horus Heresy that was to engulf the Imperium of Man and bring death and calamity to unnumbered billions, that proverbial shot was fired on the world of Istvaan III when brother turned against brother. It had only been fifteen standard years earlier that Istvaan III and the worlds within the surrounding star system was brought into Imperial Compliance by the 27th Expeditionary Fleet led by the Primarch Corax and his Raven Guard Legion. The military forces of the indigenous Istvaanians were professional soldiers who did not break easily, even when confronted with the superhuman might of Space Marines. There was some resistance, but the last elements of the aggressive faction opposed to Imperial Compliance were destroyed by the Raven Guard at the Redarth Valley on Istvaan III. An Istvaanian delegation formally surrendered before the Primarch Corax and the 800 companies of Raven Guard Space Marines that comprised his XIX Legion. They kneeled as a defeated foe, but were welcomed into the fold as men of the Imperium. Though they had waged war against one another, the Imperial Truth had prevailed and the Istvaanians had sworn to accept its teachings. By proving themselves men of wisdom and civilisation, they were deemed fitting partners for the many other worlds of the nascent Imperium. The Remembrancer Order had not yet been foisted upon the Space Marine Legions by the Council of Terra at this time, but a substantial civilian contingent was left behind to begin the reeducation of the Istvaanian population with the Imperial Truth. This task was given to Vardus Praal, formerly a Major of the XI Lastran Rifles Regiment of the Imperial Army and later a career politician of the Ultima Segmentum Court. Given Praal's record both as a soldier and civil administrator, he was deemed worthy of being left behind as the world's Imperial Planetary Governor to command Istvaan III in the name of the Emperor of Mankind, to ensure continued Compliance and manage the dismantling of the traditional religious structures that defined the planet's autochthonous society but were in direct contravention of the ancient Imperium's official atheism and secular rationalism.

Insurrection on Istvaan III
Over time, matters quickly worsened on Istvaan III, as Governor Praal ordered the Imperial garrison to tear down temples and breach the sealed vaults of ancient religious sites in order to break the subtle but deep hold superstitious fears had over the Istvaanians and bring them into the rational light of the Imperial Truth, but of the success of this campaign nothing could be ascertained as Warp Storms that continued to block Istvaan from the Imperium's core worlds worsened and all communications ceased. Six Terran years after the last official Astropathic communique from Istvaan III was received by the Council of Terra, a Death Guard patrol ship on the outer edge of its Legion's fleet, near Neo-geddon on the very edge of explored Imperial space, picked up a faint echo of an Astropathic transmission from Istvaan III. Though it was garbled and incomplete, its central message was however clear: the people of Istvaan III had risen in religious rebellion against the Imperium and the Imperial planetary government had collapsed. Vardus Praal had become a turncoat and perhaps turned mutant or psyker. Praal was actually leading the revolt, and the streets of Istvaan III's planetary capital, the Choral City, were running red with the blood of non-believers.

The message was estimated to be at least two standard years old, possible six, but it could not be ignored. For Istvaan III and its tens of millions to have rebelled from Compliance was a mar upon the Great Crusade that could not be tolerated, lest such sedition and malcontent spread, but that an anointed Imperial Commander who had received his office by the writ of the Emperor Himself led the revolt was a crime that demanded the swiftest and most resolute punishment possible. It fell to Horus, the Imperial Warmaster, to bring Vardus Praal and all of Istvaan III to judgement, and he publicly vowed to make a fearful, bloody example to all the worlds of the Imperium of the price of sedition. But what few knew or suspected at the time was that the Istvaan III rebellion had provided Horus with the perfect opportunity to strike the first blow of his own rebellion against the Emperor, a blow that would fall upon his own gene-sons.

Conspiracy
The reconquest of Istvaan III provided Horus with the perfect opportunity to both amass those forces loyal to him without suspicion, and also rid his own XVI Legion and those of his closest allies of those within their ranks whose loyalties were suspect and likely to stand with the Emperor in the event of civil war. The Istvaan System's distant location in the Ultima Segmentum and prevalent Warp Storms made it a perfect screen for the dark deeds that were to come, and orders were given for four Space Marine Legions, the Death Guard, Emperor's Children, World Eaters and the Warmaster's own Sons of Horus, to rendezvous at the Istvaan System, their approaches made to bypass the Warp Storms which raged to the galactic southwest of the system. Horus took this opportunity to use his authority as Warmaster and move the forces of the Imperial military as game-pieces on a board of regicide to the configuration of his choosing that would be most advantageous once his revolt began. He commanded the Primarchs Lion El'Jonson, Sanguinius and Roboute Guilliman to muster their Legions in preparation for a series of missions in the Signus Cluster and Veridian Systems, each in dangerous and cut-off regions of space. Perceiving no obvious reason to doubt the Warmaster's motives, the three Primarchs set about planning their missions and transporting their Legions to the necessary staging points. So it was that three of the most powerful and staunchly Loyalist Legions -- the Dark Angels, the Blood Angels and the Ultramarines, were sent by Horus to regions far from Terra and from the Istvaan System where they could not observe or intervene in what was to happen next. As for the rest of the Legiones Astartes -- the Night Lords, Alpha Legion, Iron Warriors, Raven Guard, Salamanders and Iron Hands, all were actively involved in various campaigns and events across the Imperium. Safely occupied, their time would come later, be it for corruption or destruction. The Word Bearers were already long woven into the treachery that was to follow by ties that would not become apparent until much later. The White Scars and Imperial Fists were conducting operations in the Segmentum Solar and were too close to the Emperor for Horus to chance contacting them without raising suspicion whilst the Space Wolves and Thousand Sons were already caught up in a murderous machination, the Burning of Prospero, that had been long in the making.

With humanity ignorant of the great and terrible events that were about to unfold, a massive war fleet gathered at the edges of the Istvaan System as elements of four Space Marine Legions began to arrive, and the first actions of the campaign were undertaken to destroy rebel outposts in the outer reaches of the star system as the fleet coalesced. The elite 1st Company of the Emperor's Children Legion led by Lord Commander Eidolon fought in concert with the Death Guard Legion's 7th Company under the command of Battle-Captain Nathaniel Garro against traitorous forces on Istvaan Extremis, the outermost planet within the Istvaan System. Whilst fighting against a powerful Slaaneshi psyker known as a Warsinger, Garro sustained serious injuries; crushing damage to his torso and arm as well as the loss of his right leg from the mid-thigh down. He was only saved from certain death by the timely ministrations of the Emperor's Children's Chief Apothecary Fabius. Taking stock of the desperate situation, the Lord Commander Eidolon made use of a hitherto unknown ability -- a modified tracheal implant bonded with the his vocal chords, that allowed him to produce a nerve-paralysing shriek similar to that employed by certain warrior breeds of the xenos Laer. This powerful ability killed the Warsinger and helped the Astartes carry the day, though its very existence was emblematic of the corruption that had already wormed its way deep into the heart of the Emperor's Children Legion.

The Traitors Gather
Fulgrim was delayed in coming to the Istvaan System, but the sizable task force of Emperor's Children Space Marines led by Lord Commander Eidolon made the rendezvous, and formed up with the rest of the Imperial fleet as it closed on Istvaan III. Horus' war host therefore consisted of the majority of his own Legion, plus significant portions of the Death Guard and their Primarch Mortarion, the World Eaters and their Primarch Angron, and Eidolon's smaller Emperor's Children task force. Along with three Primarchs in attendance, itself a rare event during the Great Crusade, this constituted one of the largest concentrations of Imperial military force seen since the battle for the Pargor Hith System against the Orks of Charhack almost a decade previously, comprising well over 200,000 Space Marines, although exact figures cannot be corroborated, alongside Titan support from the Legio Mortis, Legio Audax and Legio Vulpa and a number of auxiliary units.

Though accounts of what occurred next remain a matter of conjecture, it seems apparent that as Istvaan III was surrounded by the Imperial warfleet, Horus met with the two Primarchs and Lord Commander Eidolon on his flagship, the Battle Barge Vengeful Spirit. This was ostensibly done to formulate a strategy for the coming campaign, but it also seems to have been the case that the face-to-face meeting was for Horus to gauge his allies' loyalty and preparations for the coming storm. It appears that as Horus had anticipated, Angron, Mortarion and Eidolon, speaking for his absent master, had identified elements of their Legions whose loyalty to the Emperor was unshakeable or who could not otherwise be trusted to follow the Warmaster's line once the rebellion began. These loyal Space Marines would pose a great threat to Horus' conspiracy if they were not dealt with immediately, and it appears that it was only at this moment that Horus revealed to his co-conspirators his shocking plan to rid their Legions of these dissident factions.

Those sections of the four assembled Legions whose loyalty to Horus could not be guaranteed (approximately one-third of each Legion) were commanded to prepare for an assault on the planet beneath and would form its core. The Warmaster's plan called for a primary strike conducted almost entirely by Drop Pod with limited gunship and teleporter support, with only the Death Guard forces landing with any significant armoured vehicle contingents in the first wave. So separated, and with no means of escape off the planet, those Loyalists who did not die in the taking of Istvaan III would discover to their horror that Horus planned to make it their graveyard at all costs.

Onslaught
"Strike fast. Cut them down. Soak the ground with their blood. Grind them to nothing as they did their oaths."

- Blood Champion Helkor, Hammer of the World Eaters, before the assault on the landing grounds of the Zotiel Basin

For the assault on the sprawling proto-hive city of Khry Vanak, Istvaan III's political and cultural capital (called the "Choral City" in translation into Low Gothic), four primary target zones had been identified which housed the seat of government, and presumably where the Traitor Vardus Praal might be located. This would be the focus of the Emperor's Children's attack. It was in the plazas which adjoined the palace and formed the principal arterial intersection of the city's transport network that resistance was likely to manifest quickly, and this was given as the World Eater's target to first seize and then spread beyond. The western fortification wall with its Imperial-built bastions and the bunker network they protected were to be the target of the Death Guard's assault, with the goal of either eradicating or boxing in the garrison forces they contained.

The final, and perhaps most difficult, target was located in the east of the city; a vast and ancient complex of tomb-spires, shrines and temples known by the indigenous peoples as the Siren Hold. Given the assumed religious forces of the rebellion and the psyker-witches known as the Warsingers also encountered on Istvaan Extremis, the Siren Hold too was afforded that status of primary target and the Sons of Horus assigned to the attack orders to destroy everything and everyone they found within. A score of secondary targets to be hit in the first wave were also identified, including the star port and several prominent infrastructural complexes and numerous smaller, splinter forces assigned to take or destroy.

Legion of Death
The first assault wave when it came showered Istvaan III with an iron rain of Drop Pods, which smashed into the Choral City like hammer blows, while strategic lance-strikes from the orbital fleet hit ground targets across the surface of Istvaan III, ensuring total air-dominance for the Space Marine Legions. Resistance was heavy, far heavier than the initial invasion had seen, but against such a force as the Space Marines now unleashed against this world, ultimately futile. Against the walls and trench works of the western fortification line the Death Guard implacably hammered home their attack against troops that had once been loyal to Terra but were no half-insane, mutilated things. They were armed with Imperial weapons taken from the garrison stores, their trench lines anchored with mortar nests and emplaced Autocannon set up with interlocking fields of fire with all the lethality of Officio Militaris expertise. Although reeling from their initial assault, the Traitors met the Astartes with Basilisks and Malcador heavy tanks which rumbled up from concealed emplacements beneath the ground. But Mortarion's Legion was indefatigable and came down right within the cordon of fire so that the Istvaanian's artillery was left firing blind and overshooting the drop zones.

With shells bursting around them the Death Guard moved in relentlessly, Terminators and heavy support squads toppling guntowers and punching their way into bunkers, while Tactical squads stormed trench after trench, sweeping them of life with methodical volleys of Bolter fire. Gunships roared in and dropped Vindicator and Land Raider tanks to assail the bastions and high-walled revetments of slab-like granite that shielded the city from the plains to the west, and the close barrage from both sides of the wall soon took its toll. Soon the great bastions were left in smoking ruins, their fate sealed as a maniple of Titans from the Legio Mortis made earth-shaking groundfall to the west of the city and unleashed the awesome destructive force of their turboblasters and gatling cannon against the wall, pulverising battlements and causing granite to run like molten wax.

Battle for the Precentor's Palace
The battle for the Precentor's Palace and its environs was a chaotic slaughter. The World Eaters attacked enmasse, via drop pod assault, striking first as they smashed into the open plazas like a meteor storm, leaving the vast stonework colonnades and terraces a shattered wasteland. As thousands of World Eaters poured fourth they killed everything in their path, overwhelming disordered garrison troops in a tide of howling violence, cutting down hundreds in mere minutes as they spread out like a swarm devouring insects. As the din of carnage engulfed the city centre, a disordered roar born of countless screams, battle-cries, thunderous bolt shots and the growl of uncounted chainblades hacking into flesh, fresh golden-fire streaks came down in tight order, aimed like a hunter's bolt at the Precentor's Palace itself. Rebuilt by the Imperial military architects to be both an imposing symbol of Imperial authority and a last redoubt in case of civil insurrection, the Precentor's Palace stood as a tried and tested piece of military architecture designed to resist, if not defy, bombardment.

The cordon of defence for the Precentor's Palace was built as proof against direct ground assault. It was a defence the Emperor's Children's attack plan fully intended to circumvent. With a precision landing vector few Legions could have matched, the Emperor's Children wave landed directly on their target, clustering around its sprawling scatter of entranceways and admission halls. With speed and ruthless force bred from the zeniths of training and pre-planning the Legion was famed for, already on top of the inner defences the Emperor's Children fought their way into the Precentor's Palace from a score of entry points against fierce resistance, while Dreadnought siege-claws and Legionary Melta Guns hacked and burned new ways in from the level of the doomed roof above.

The Siren's Hold
Across the city, with the sundering force of the claw-strike of a great predatory beast, the Sons of Horus drop pod assault descended upon the Siren Hold. Larger than the precincts of the Precentor's Palace, the Siren Hold was a scatter of great irregular conical tomb-spires, in appearance almost like deep ocean smoke-stacks, jutting hundreds of metres into the air. Scattered and under heavy fire from the outset, the Sons of Horus nevertheless rallied to the attack with their customary discipline and battle-lust, and the Siren Hold quickly became a tortuous, multi-storey killing ground, filled with shrapnel and smoke. It was here the Warsingers made their first appearance in strength. These psyker-witches were able to manipulate sound by the means of strange relic-technology to take to the air and surround themselves with bubbles of shielding force. Their screaming songs were able to shatter the ceramite armour of the Space Marines and rupture the flesh and powder the bone within, and drove the warrior-cultists that defended them to inhuman levels of fanaticism so that they hurled themselves at the Sons of Horus heedless of the shredding bolter fire that met them.

As the battle was fully joined the death toll mounted staggeringly as a great paean of howling sound engulfed the Choral City, drowning out even the din and cacophony of battle - an unnatural scream that tore at the brain and sent the people of Istvaan III into shuddering paroxysms of uncontrolled hatred. It was an auditory barrage that only the Space Marines' willpower and auto-sense baffled armour allowed them to resist and fight on regardless of the strange assault. At the western walls and trench works the Death Guard and the Legio Mortis had left the great bulwarks in rubble and now the Legionaries were brutally sweeping the bunkers and tunnel networks beneath clear of defenders, while in the plaza area the World Eaters were surrounded by a sea of slaughter. Whatever dark power drove the terrified populace suddenly drove them forwards towards their attackers, heedless of their lives as they came at the World Eaters, first in dozens, then hundreds and then thousands. Unarmed and unarmoured, the screaming, laughing human waves recklessly came to Angron's already blood-spattered warriors to die, and the World Eaters were keen to oblige. As wave after wave came at the World Eaters they were pushed back by simple overwhelming weight of numbers. Warriors fought back to back, sending Frag Grenades into the mass of densely packed bodies hacking and slaying until they were left with ramparts of dripping meat. The XII Legion's heavy support squads ran their weapons dry cutting great reaping swathes through the onrushing crowds with their Heavy Bolters and Missile Launchers.

In the tangled warrens of the Siren Hold and the inner walls of the Precentor's Palace victory was being clawed from the enemy a metre at a time, but not without cost. The unnatural psyker-powers of the Warsingers slew many before they were brought down, and elite warriors of the rebellion's inner-sanctum were encountered - surgically modified and mutilated, clad in black vitrified armour and bearing strange and hitherto unknown relic weapons that spat killing blasts of sound or darts of liquid metal. Deep within the Tomb Spires, the Sons of Horus spearhead, under the command of senior Captain Garviel Loken fought their way into a strange, corpse-filled shrine of foul idolatry, later named Death's Tomb, and slew the Warsingers there, while at roughly the same time, although having suffered heavy casualties, a small strike group of Emperor's Children under the command of the famed swordsmen Captain Lucius fought their way into Vardus Praal's throne room and confronted the traitor.

Praal, armoured in twisted baroque splendour and wielding strange psycho-sonic weaponry, was indeed master of this strange and arcane cult rebellion, but even his augmented might was not enough to best Captain Lucius who slew him in desperate battle. Regardless of whether Praal's death served to cut the head off the serpent, or if the destruction of the vile shrine at the centre of the Siren Hold was its cause, the strange cacophony that had held sway over the city collapsed and with it the power that had maintained the Istvaanians' resistance was ended. Within hours the rebellion was in tatters with all of the invasion force's objectives secured and tens of thousands of the enemy dead in the wake of the assault. The victorious Space Marines were jubilant, their victory hard-won and well-deserved, but unbeknownst to them, disaster was about to strike.

Betrayal
"Order the guns to fire," said Horus, his voice cold. "Let the galaxy burn!"

- Warmaster Horus during the virus-bombing of Istvaan III

Shortly after the battle had turned in the Space Marines' favour, all communications with the orbiting fleet had ceased and the Titans of the Legio Mortis began a silent and implacable withdrawal beyond the city walls into the empty plains beyond. With inexorable slowness the great battleships and battle barges of the combined fleet; Vengeful Spirit, Firebird, Andronius, Killing Star, Indomitable Will, Gauntlet of Spite, Warchild, and Conqueror descended into low orbit and began saturation bombardment of the planet. Orbital bomb racks bloomed and tiered decks of macro-cannon batteries unleashed torrent after torrent of shells downward into Istvaan III's lower atmosphere. The deadly cargo was the Life-eater Virus, one of the most terrible Exterminatus-class weapons in the Imperial arsenal, and a weapon whose use only the Warmaster and the Emperor could have commanded. Dispersed by concussive charge from a thousand exploding munitions, the life-eater virus began its work in moments, infecting and destroying, a rapidly-spewing necrophage which turned every living thing it came into contact with to sickening, liquid rot. Human flesh sloughed from bones and screams were cut off as victims drowned in the decaying fluids of their own lungs. Plant life withered and melted into brown-black sludge as if time had turned them to the aged blight of years in mere minutes. Black gales of rot and corpse vapour moaned through the concrete and steel canyons of the cities, sounding the death knell of eight billion souls. Beyond the graveyard cityscapes the wide plains and jungles collapsed into cankerous decay, while the oceans turned to greenish sludge so that to the war ships of the Warmaster's fleet above it appeared that cancerous stains rapidly spread across the surface of the planet until it hung like rotten fruit against the blackness of the void. Istvaan III died. The psychic shock of so many deaths at one time shrieked through the Warp, briefly obscuring even the Astronomican.

But the destruction was not over. An entire planet’s worth of decaying matter wreathed the atmosphere of Isstvan III in a thick shawl of combustible gasses. All it took was a single burning lance strike from the Vengeful Spirit to set the planet aflame. In a second, the air itself caught light, ripping across the landscape in a howling maelstrom of fire and noise. Entire continents were laid bare, their landscapes seared to bare rock, their decayed populations vaporized in seconds as winds of fire swept across their surfaces in a deadly gale of blazing destruction. Cities exploded as gas lines went up, blazing towers of fire whipping madly in the deadly firestorm. Nothing could survive and flesh, stone and metal were vitrified or melted in the unimaginable temperatures. Entire sprawls of buildings collapsed, the bodies of their former occupants reduced to ashen waste on the wind, palaces of marble and industrial heartlands destroyed in gigantic mushroom clouds as the storm of destruction swept around Isstvan III with relentless, mindless destruction until it seemed as though the entire globe was ablaze. Those Astartes who had survived the viral attack found themselves consumed in flames as they desperately sought to find cover once more. But against this firestorm there could be no cover for those who had dared to brave the elements. By the time the echoes of the recoil had faded on the Warmaster’s flagship, billions had died and Istvaan III had become a dead planet. Warmaster Horus had at last declared his hand and openly defied the Emperor, and so consigned the galaxy to civil war. He had also planned that the virus bombardment would destroy a host of Space Marines he could not trust to fight for hm in the coming struggle with the Emperor, but as was soon to become apparent his plan's perfection had failed.

Wrath & Ruin
"This Act of Heresy shatters all ties of kinship. Those I was proud to call Brother I now fight to bloody death. No quarter will be asked and none sought. These are the vilest fores we must fight. There shall be no peace, no rest, no succour until the face of Horus stares down from Traitor's Gibbet and all his heretic followers are vanquished."

- Anonymous Loyalist, Istvaan III Atrocity

Even before the first treacherous blow struck, the Warmaster's plan had, unknown to him, begun to unravel. Before the bombs fell, the impending atrocity had been discovered by several among the fleet who remained loyal to their Emperor and their comrades and who resisted. Chief among these was Battle-Captain Nathaniel Garro of the Death Guard, a Terran veteran and one of the original recruits of the XIV Legion. Garro acted swiftly, rallying Loyalists to him and successfully commandeering the heavy frigate Eisenstein, and signalling warning messages to his battle-brothers on the planet's surface. Elsewhere amid the Emperor's Children fleet, Captain Saul Tarvitz, a line officer of his Legion who had long harboured deep suspicions of his Legion's growing secrecy and actions, had uncovered the truth mere minutes before the onslaught was to begin. Powerless to prevent what was to come, Tarvitz stole a waiting Thunderhawk gunship and descended to the planet's surface, carrying his own warning of the horror that was about to be unleashed. Those that heard or passed on Tarvitz's warning took shelter before the virus-bombs struck. The World Eaters on the planet's surface were the furthest from cover and suffered the worst of the initial bombardment. Some had reached the safety of the bunkers, but many more had not. Warriors fell to their knees as the virus penetrated their armoured bodies, deadly corrosive agents laced into the viral structure of the weapons dissolving exposed pipes and armour joints, or finding their way inside through battle damage. Astartes screamed. The sound was all the more shocking for its very existence rather than for the horror of its tone. The virus broke down cellular bonds at the molecular level and its victims literally dissolved into a soup of rancid meat within minutes of exposure, leaving little but sloshing suits of rotted armour. Even many of those who reached the safety of the sealed bunkers died in agony as they shut the doors only to find they had brought the lethal virus inside with them. When the firestorm began after the initial bombardment, those Astartes who had survived the viral attack found themselves consumed in flames as they desperately sought to find cover once more. But against this firestorm there could be no cover for those who had dared to brave the elements.

Receiving no such warning, others were not so fortunate however, and many Loyalists and suspected Loyalists not assigned to the ground attack looked on in shock as the virus bombs fell, only to find knives waiting for their backs. Many resisted valiantly but, caught utterly unprepared for such perfidy, were destroyed. This fate not only awaited Loyalist Space Marines but others, including large numbers of human auxiliary troops, support staff, iterators and functionaries who, caught up within the ships of the fleet, were massacred as teh Legionaries loyal to Horus in their midst turned on them with merciless violence. Even here however not all went to plan and the "mere" humans, in many cases underestimated by the superhuman Legiones Astartes, put up considerable resistance or fled into the bowels of the great ships and had to be hunted down at length, often carrying out campaigns of sabotage in the process.

During the orbital attack at least one torpedo-monitor, the Ducroix, remained in Loyalist hands and turned its firepower on the Traitor ships in a futile attempt to interdict the bombardment, destroying several escort ships and inflicting significant damage on the Battleship Killing Star before it was ripped apart by traitor fire. Having refused the order to surrender, the Mechanicum Ordo Reductor Galleass Xerxes 9-7-7, a vessel that had joined the fleet late and was not part of the Warmaster's plans, also had to be shot down at considerable cost to its attackers, its disintegrating hulk descending into Istvaan III's burning skies and lost amid the flames. As this was occurring on the fringes of the fleet cordon the heavy cruiser Sunstone was the scene of a protracted on-board struggle. Here the sworn lifeguard of the Terran Plenipotentiary, the renowned Duke Mortecher, turned what should have been a simple takeover of the ship by a delegation of the Emperor's Children to assassinate the Council of Terra's political emissary, into a frantic deck-by-deck battle for control of the vessle. It is not known what was to finally occur aboard the Sunstone, but as the firestorm raged on Istvaan III below, the Terran warship suffered a catastrophic detonation in her forward magazine, leaving the Sunstone a splintered wreck.

Flight of the Eisenstein
"His dreams and hopes are in ruin, his trust proved false, and his brightest son fallen to darkness. These are bitter tidings, a nightmare made flesh, but they must reach the Emperor."

- Nathaniel Garro, former Battle-Captain of the Death Guard

As this maelstrom of confusion and destruction was unfolding, Battle-Captain Garro knew he must somehow make good his escape and bring word of Horus' perfidy to the Emperor. But Garro knew his lightly armed Frigate was no match against the powerful capital ships of Horus’ blockading fleet. At the time, the Eisenstein was close to the rear edge of the fleet pattern. The ship’s captain took the liberty of informing the fleet master’s office that they were suffering a malfunction in one of their tertiary fusion generators. It was standard naval procedure for a ship under those circumstances to drop back from the main formation, to prevent other vessels being damaged in case of a cascade failure and core implosion. But the Loyalists knew that the ruse would not last for long. For they would be undone the moment they fired their main engines.

Realising something was amiss, Death Guard First-Captain Calas Typhon aboard the massive Death Guard Battleship Terminus Est moved to intercept the small Frigate when he had received no word from his usually boisterous subordinate Commander Ignatius Grulgor. Typhon gave chase as the Eisenstein attempted to evade the guns of the formidable vessel. The frigate sustained severe damage from the Terminus Est's massive guns batteries as it sped past. The crippled ship limped away from Istvaan III. It was severely damaged, and all of its Astropaths had perished in the firefight and their lone Navigator was mortally wounded. The ship was incapable of interstellar communication and had little chance of successfully navigating its way across the Immaterium, but Garro ordered the vessel to make a blind Warp-jump in order to save the lives of everyone aboard. The Eisenstein managed to break free of the fleet and escape at maximum thrust, managing an emergency translation to Warp space and fled the Istvaan System, its fate uncertain for its pursuers. This single act of heroic defiance was to alter the course of all that was to follow, but it would not be the only reversal of fortune the Traitors were to suffer.

Survival
"We are the Sons of Horus no longer! That name has no meaning to us now. We are the Luna Wolves, Soldiers of the Emperor, and for him we will fight and if needs be, we will die gladly!"

- Garviel Loken, Captain of the Luna Wolves, Istvaan III

As the firestorm below guttered out, as it had been designed to do, surveyor-sweeps and Auspex beams were eagerly focused downwards from the orbiting fleets with the expectation of registering the charred remains of a dead and lifeless world, but quickly shock turned to rage for the Warmaster. On the planet's surface had been nearly 100,000 Space Marines drawn from the Emperor's Children, Death Guard and World Eaters Legions and they had taken Istvaan's Choral City with less than a tenth of their number as casualties. Although the full extent of their failure was not yet apparent to the Traitors, perhaps fully two-thirds of the first wave had miraculously survived the bombardment, thanks to the warning messages they received from their loyal comrades in orbit. The Loyalists had found shelter in hastily resealed bunker complexes they had only hours before taken from the Istvaanian defenders, or found protection in the stormed bastions of the Precentor's Palace or in the kilometres of catacombs which threaded out from under the Siren Hold. Warned just in the barest of time they had trusted to luck, to training, to wargear and the physiology of the Emperor's great design, and they had endured.

As the great storms which came after the firestrom abated, communication channels crackled into life and frantic signals rippled across the surface of Istvaan III and stabbed skywards demanding answers, screaming defiance and spitting curses against those the betrayed had once called brother. Never in the history of the Legions had there been such a terrible act of treachery and malice and the Space Marines on Istvaan III were incensed beyond reason, some driven to the point of madness, that their own Primarch most of all, had betrayed them.

Lord of Wrath
"And the sky burned, and the stars fell from the inferno, and with them came the angels, And they were armoured and arrayed for slaughter, and their rage howled across the ashes of the realm they had destroyed. And with them was one greater than all the rest, they his dead children and he was their maker. And those that remained amongst the ashes looked to the sky and knew that death was come upon them."

- The Lucid Visions of the Oracle Menacus, Eighth Book, 64th Chapter

In the wake of the failure of the virus bomb attack to annihilate the Loyalists, matters further spiralled out of hand. Even as Horus attempted to reestablish strategic control of the situation and order a second, conventional bombardment, against orders and perhaps goaded by the response of those in his service, or as it has been suggested since, driven to grant the honour of dying by his own hand to those who once served him, the Primarch Angron took matters into his own hands. Spearheaded by a wave of gunships and followed by a second drop pod wave, the World Eaters descended from the mighty Conqueror with Angron himself and his chosen warriors at the fore. The Warmaster and his allies could only look on with outrage as the Primarch of the World Eaters made planetfall with a full 50 companies of his bloodthirsty Space Marines at his back, landing in the plaza areas to the west of the Precentor's Palace, hunting for his own kin with fratricide in their hearts. Their Thunderhawk and Storm Eagle gunships, strafing the now ash-strewn ruins, were met immediately with a sporadic return of fire, they had found their intended prey.

The "Red Angel" roared from the belly of his gunship onto the blackened bones that littered the plazas like fallen leaves in a winter forest, his great chainaxes Gorefather and Gorechild screaming in anticipation of the slaughter to come. His challenge so issued was quickly met as storms of weapons fire licked out form the shattered cityscapes to find him, but he strode through it heedless like a monster of dark myth made flesh, Behind him his World Eaters formed up in a great arrowhead of pale ceramite and glittering metal, eager for the slaughter to come, the forbidden rage-induced implants, known as the "Butcher's Nails," dug into their brains screaming the need to kill with cruel and irresistible force. Inevitably, from the line of shattered ruins that edged the plazas, their loyalist brothers came, no less World Eaters, no less consumed by rage, a rage now fuelled to greater heights than they had ever known by the bile of tasted treason. Brother-Captain Ehrlen led them, a warrior whose tally of victories had been long in the service of the blood creature he now faced. With him were a force of Loyalist Space Marines some 2,000 strong, a core of World Eaters who had sheltered with their commander in nearby garrison bunkers to wait out the storm of what they had been told (for believability's sake) was an Istvaan suicide bio-weapon about to be unleashed, the rest of their Legion contingent shattered far and wide across the cityscape. For Ehrlen and his men the truth had been almost too terrible to face, and here and now, confronted by two-and-a-half times their number of those they had once called brother and the Primarch to whom they had pledged their lives, all set against them, it was simply too much. A homicidal madness overtook them, they would make an end of it, dragging down as many with them into death as they could claw and cut the life from.

Both sides charged headlong, without tactics and without reason they charged into the arms of death, crashing against each other like competing waves of force. In moments hundreds were slain as superhumans marched spite and fury against superhuman. Ehrlen was overwhelmed by dozens of World Eaters and dismembered, still spitting curses at those he once called brothers, while charging Traitor Dreadnoughts exploded in the converging furnace heat of multiple melta blasts, showering shrapnel and flame across the battle lines. In the carnage that ensued a truth that was to become a bitter reality for years to come sprung fully formed into the reckoning of war; when Legiones Astartes fought Legiones Astartes the usual Imperial logic of battle, of superiority of force, of an enemy's morale broken, of belligerents weighed and found wanting, no longer applied. Instead matched in power and weapons, armour and skill, and above all in the unshakeable resolve to fight, the result was an appalling stalemate of expended force and savage attrition. Battle would now be characterised by superhuman bodies hacked and blasted until they could take no more, of weapons emptied in wrath and battle plate pulverised by not one blow but a score's counting before its wearer succumbed, but succumbed with the foe's blood on their hands as well. This would be a war in which no victor would escape the battle unscathed.

In the slaughter of the Choral City plazas where 7,000 World Eaters smashed and butchered each other in a frenzy of violence, it would be the Traitors' numbers which finally turned the tide, that and the presence of their bloody lord. Angron was a machine made for killing, an unstoppable engine which sent torn bodies and shredded limbs tumbling away in looping arcs of crimson blood and spraying sparks of rent metal. He careened like a thunderbolt back and forth through the clashing press of armoured bodies, destroying everything that stood against him, shrugging off blade-strokes and bolt shell strikes as if they were the mere bite of insects, roaring his challenges and calling out the names of the worthy dead his axes reaped. The killing continued until there were none left to kill and the ravaged victors, their once-white armour now a gory red, stood amid a morass of shattered bodies, the filth of blood and clotted bone-ash around them like a spreading strain across the world. Upon viewing the horror that had been wrought, Angron is said to have exclaimed, "It is well done!" He then ordered his World Eaters to spread out through the city and seek more survivors to kill. So began the true bloodshed of the first inter-Legionary war, the first battle in the Imperium's history when its greatest defenders, the Space Marines, turned upon themselves in battle without quarter asked or given. Such was the shocking betrayal, the pain and price of treachery so great, that there could be no respite or relenting in such a battle, only bitter hatred and bloody vengeance.

The Traitors Strike
"There is no failing I cannot turn to a component in my victories. Angron has decided to turn Isstvan III into a ground assault – I can consider this a failure and limit its impact by bombing Angron and his World Eaters into dust along with the rest of the planet, or I can forge a triumph from it that will send echoes far into the future."

- Warmaster Horus

The Warmaster was incensed with Angron's disobedience and disruption of the carefully laid plan, a plan now almost in ruins. In retrospect it is clear to see the invidious position the World Eaters Primarch had placed Horus in. He could have continued with his preparations for a second bombardment and indeed carried it out, firing on both the Loyalists and Angron to punish him, but that at best would see the World Eaters' fleet turn on Horus' allies and start a second civil conflict the Warmaster could ill afford, and at worse, leave Angron alive and an enemy. No, there were far too many Loyalists left on the surface to leave to the World Eaters, and if they were not attacked soon they would become dug-in and organised, and therefore infinitely more dangerous, while bombardment was no longer a viable option. The Warmaster was committed to fight and left to forge victory from disorder. So it was that Horus had no choice bu to back Angron's headlong assault, and the general order to prepare for ground attack was given.

Once the die had been cast for a brutal war on the ground, the arch-traitor Horus appeared to have made a decision to turn the situation to a wider advantage in regards to the loyalty of those that had pledged to fight at his side to usurp the Throne of Terra. Rather than utilise a single Legion to press the attack to the Loyalists' destruction and spare his other forces intact for later battles, he chose instead to employ each and every division at his command to some degree in turn. In this Horus clearly showed his wider grasp of the greater scale of war to come, ensuring every element of his command was bloodied in the field against their former comrades, with the deaths of the Emperor's Space Marines laid squarely at their door. With this pact sealed in blood, there could be no going back for those that had turned against the Emperor and no forgiveness, only final victory or death.

The Emperor's Children under their commander Eidolon were next to deploy, no doubt smarting over the actions of the Loyalist Saul Tarvitz and keen to redeem themselves in the eyes of the Warmaster. With typical precision and exacting patterns of deployment, the traitor Emperor's Children selected a scorched-flat area formerly a open markets and parks a few kilometres south of the Precentor's Palace for their staging area. No sooner had Eidolon's taskforce dismounted their conveyers then he ordered his forces to immediately precede in column formation to the Precentor's Palace with the intention of taking it by storm and overwhelming what he clearly assumed would be disorganised and broken group of survivors. Meanwhile further World Eaters support forces started to land, preparing for a lengthy battle of attrition, while Death Guard and Sons of Horus gunships began to conduct reconnaissance sweeps to the northwest and south of the Choral City respectively, searching for survivors amid the still burning ruins, firing on targets of opportunity and dropping off Seeker squads and outriders to hunt down isolated groups of Loyalists still reeling from the planet-kill attack. Within hours however it became clear that this would be no simple mopping up operation for the superior traitor forces.

Eidolon's column found the Precentor's Palace staunchly and expertly defended by the Loyalist Emperor's Children forces who Eidolon had arrogantly underestimated. Instead of succeeding in a storm assault, his column came under heavy fire and was blocked from even reaching the palace walls and forced to retreat, leaving dozens of damaged and wrecked vehicles in its wake. Further attempts to send infantry squads through the ruins fell afoul of hastily set-up but effective redoubts and pre-arranged ambuscades, hurling his men back bloodied and without gain. Elsewhere the use of roaming gunship detachments had proved equally ineffective; auspex sweeps in the burning, tangled ruins could provide no more than a vague indication of enemy concentrations, while the vast blasted cityscape was a perfect haven for the enemy to lie in wait and loose missiles and lascannon blasts at the low-circling aircraft, the particular vulnerabilities of which were completely familiar to the Loyalists as they were their traitors attackers, such was the price of brother fighting brother. Worse, one Sons of Horus Storm Eagle retuning to the bay of its mother-ship, the Strike Cruiser Minotaur and displaying proper codes had conducted a suicide attack inside the launch bay, voxing its intentions to, "...avenge the blood of Terra" before ramming itself into a line of rearming gunships, lascannon blazing, wreaking carnage and killing a full company of Horus' Legionaries in the process. Comm-channels and vox-codes already switched when teh attack began were hastily being changed again, causing further disruption as the Minotaur incident added to a death toll that was already rising rapidly and shockingly in the Loyalists' favour.

War of Attrition
As night fell, the debris and dust thrown into the upper atmosphere began again to descend, the bleeding heat of the dying world birthing fresh continent-spanning storms which ravaged the tortured air of Istvaan III and closed over the Choral City with renewed force. Hurricane-force winds drove great torrents of still-hot ash through the ruined canyons of city-blocks and tomb-stone structures peopled with the dead, while thunderous volleys of lightning played havoc with vox-transmissions and surveyors. Flight was virtually impossible and the surface all but cut off from the fleet. While the first stage of the plan, the separation of the Loyalists had proved successful, the second phase was approaching unmitigated disaster, a fact that mus have been readily apparent to the master-tactician that Horus undoubtedly was. Even masters of their craft however, post-human and human alike, are subject to the whims of chance and action. The virus-bombing of Istvaan III had ultimately done nothing more than slaughter billions of innocent bystanders and turn Istvaan III into a hellish warzone of rubble, ash and storm which massively advantaged the defender while it vexed an attacker in turn.

The overconfidence of the Emperor's Children had brought failure, and Angron's World Eaters had dispersed into the northern city in an uncontrollable and untrackable horde. Probing sweeps by aircraft had done little but suffer casualties, and revealed the scale of the Loyalist resistance and demonstrating their bitter resolve to fight. The bitter fighting on Istvaan III saw the outnumbered Loyalists seeking to equal the strategic odds in their favour by dragging their enemies into conflicts in built-up areas and ruined landscaped where the full panoply of their Traitor foes could not be brought to bear upon them at once, fighting running battles through corpse-strewn streets and shattered habitation blocks, blackened by fire and ash. Hundreds of hostile contacts had been flagged right across the sprawl of the shattered city, with particular concentrations identified in the northwestern zones, the Precentor's Palace area and the lethal tangle of the Siren Hold district. Horus and his commanders must have had no doubt that under the cover of the darkness and the storm the Loyalists were on the move through the ruins and underneath them, through catacombs, sewers and tunnel networks, consolidating, organising and fanning their lust for vengeance to a murderous heat.

What was to come was to be a gruelling war of attrition where air power would count for little, and even though Horus had the Titans of Legio Mortis at his command, he could not risk them in the city-ruins while the storm raged, blinding their scanners and defeating their advantage at range. Days became weeks as the tempest went on all but unabated and beneath its cover, bitter close-quarter battles were waged between Loyalist and Traitor in the ruined city, the forces almost evenly matched in numbers. In the gaunt frameworks of habitation blocks and in the fetid darkness of sewer networks thick with corpse-waste, Space Marines battled Space Marines unto the death. The Traitors could only reinforce their assault piecemeal thanks to the storms, and during the worst of the climatic upheaval were forced into supplying munitions by drop pod if at all. In the wrecked industrial zones of the southern city armoured columns of Vindicators and Predator tanks made grinding progress, battling block-by-block against brutal infantry counter assault and suffering a toll exacted by snipers and heavy weapons teams hidden in the twisted morass of factory ruins and toppled smoke-stacks. In the twisting shadows of the Siren Hold, warriors who had once proudly displayed the colours of the Warmaster and called themselves Sons of Horus defaced their armour and heraldry and reclaimed the name of the Luna Wolves once more. Their leader was Garviel Loken, once a hero of his Legion and commander of its 10th Battle Company, and formerly part of Horus Lupercal's inner circle, the Mournival. Singled out by the Traitors for his unwavering loyalty to the Great Crusade and its ideals, Loken had been betrayed to his death on the surface like the rest, but now rose to the mantle of general and leader, coordinating resistance in the eastern and southern sectors of the city and masterminding a mobile defence of counter-thrusts and hit-and-run attacks that stopped his erstwhile comrades from employing the lethal encirclement tactics at which they excelled. Again and again Eidolon's Emperor's Children attempted to take the great granite bulk of the Precentor's Palace only to be hurled back by Captain Tarvitz and his warriors, displaying implacable resolve and tactical excellence in their defence of the shell-shot edifice and its precincts. The palace was now further reinforced with recovered heavy weapons and patch-repaired vehicles claimed from the spoils of Eidolon's first failed attacks, and with the Loyalist Emperor's Children fought the mighty Ancient Rylanor, a Dreadnought of the III Legion, and to them he was like an undying icon of past glory and honour the Traitors had so hatefully cast aside. Many times was the venerable dreadnought thought lost, only to rise again from the rubble and fire to punish those that had betrayed the Emperor whose name they bore. The Traitors' attacks against the palace were further blunted by the need to guard their own flanks in the dust and darkness, as roving war bands of World Eaters, most Loyalist but some perhaps merely uncaring of their "allies" lives attacked from the ruins and rubble, slaying and plundering arms and munitions before being cut down by discipline fire or disappearing victorious back into the darkness.

To the northwest, the Death Guard Loyalists faced the direst situation of battle. Although heavily dug into the Istvaanian III bunker networks, trench lines and the screening city walls, and well-supplied with arms and munitions taken from the underground vaults of the planetary defence armouries, they were cuaght between teh wrath of two implacable foes. From the inner city came Angron and his World Eaters, hammering again and again against their defences, hacking their way into tunnel networks heedless of casualties taken in the breach and leaving everyone within a red ruin of flesh and shattered ceramite. Beyond the city wall Mortarion, as implacable and relentless as death itself had braved the storm and landed the great relic-assault ship Omen amid the howling gales. Showing far more respect for the foe he faced than the traitor Emperor's Children had, Mortarion had Omen's turret guns sweep the ground clear, smashing the nearby trench networks and bunkers flat so they could provide no hiding place for a Loyalist ambush before unleashing its cargo. In defiance of the hurricane and the darkness, the Death Guard marched forth in ordered ranks, and with the mthe Legion's behemoths of war, Spartan heavy assault transports and Fellblade and Typhon super-heavy tanks, and began their systematic and relentless attack with the Reaper-Primarch marching slowly at their head, counting the slain.