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Isstvan V, also spelled as Istvaan V in Imperial records, was the Dead World in the Isstvan System proximal to the Segmentum Obscurus where the infamous Drop Site Massacre of the Horus Heresy took place in ca. 006.M31 that deeply damaged the Loyalist Space Marine Legions in the worst betrayal in Imperial history.

During the massacre, the Traitor Legions of the Warmaster Horus redeployed following the virus-bombing of the Traitor Legions' own Loyalist members at Isstvan III at the start of the Horus Heresy during the Battle of Isstvan III.

Upon learning of the terrible atrocity Horus had committed from the survivors of the frigate Eisenstein, Rogal Dorn, the Praetorian of Terra, ordered seven Loyalist Legions of Space Marines deployed under the Emperor's authority to the Isstvan System to bring Horus to account for his actions.

The Iron Hands, Salamanders and Raven Guard made up the first wave of the attack, and were making significant progress against Horus' maddened Chaos Space Marines.

Then the Loyalist reserves were called in to finish the job, but the four supposedly loyal Space Marine Legions comprising those reserves -- the Iron Warriors, the Alpha Legion, the Word Bearers and the Night Lords -- had also secretly betrayed the Emperor and were prepared to swear themselves to the service of the Warmaster and Chaos.

The three Loyalist Legions were almost annihilated in the resulting crossfire from the eight revealed Traitor Legions but several thousand survivors from each of these so-called "Shattered Legions" managed to escape off-world, though they were too decimated to play much more than a supporting role and conduct behind-the-lines guerilla strikes in the defence of the Imperium from Horus' Traitors.


The Great Crusade

The Isstvan System was unusual in having several different worlds within tolerable ranges of Human life, as well as several autochthonic species of life with similar genetic origin, leading the Magos Biologis of the Mechanicum to classify it as being the likely subject of past terraforming in the age before the coming of Mankind into the galaxy.

At the time when the Great Crusade reached the system in the late 30th Millennium, the third planet of the system (designated Isstvan III) was the site of a thriving indigenous Human population which had taken root during the Age of Strife, overbuilding a long-dead xenos civilisation of an unknown origin.

Rendered barren many Terran millennia earlier after a long period of tectonic instability and volcanism, Isstvan V still showed signs of original xeno-sentient occupation, particularly in the fragmentary remains of continent-spanning road networks, shattered dam-like structures over long dried-out watercourses and, most prominently, certain remnant macro-fortifications.

These blind and empty shells were devoid of any maker's marks or the evidence of the advanced technology which had undoubtedly been used to produce them, but the molecularly-dense artificial stone of their construction remained as a testament to the power and arts of their long-dead creators.

These towers and redoubts stood silent vigil on a cold world empty of life save for lower orders of hardy flora, and a post-volcanic landscape made up of hulking, irregular mountain ranges, broken deserts of black sand, weather-beaten mootland and a few stout, land-locked seas of sulphurous water.

The Drop Site Massacre

Aloni Falcons

Astartes of the Raven Guard fight for their lives on Isstvan V.

At the start of the Horus Heresy in the 31st Millennium the Warmaster Horus apparently only had 3 Primarchs and their respective Space Marine Legions at his side. Under his command were his own Sons of Horus, joined by Angron and his World Eaters, Fulgrim with the Emperor's Children, and Mortarion and the Death Guard.

After ridding himself of all suspected Loyalist members inside the three Traitor Legions during the virus-bombing of Isstvan III, Horus chose the other habitable world in the Isstvan System, Isstvan V, as his command post and prepared a deadly trap for his Primarch brothers and their Loyalist Legions.

The Emperor of Mankind ordered the deployment of seven full Space Marine Legions against Horus and the known Traitor Legions, agonizing over the betrayal of his most beloved son. Unknown to the Emperor, four of the deployed Primarchs and their Legions had already turned against him and sworn themselves to the service of Chaos, forming a "fifth column" which would strike against the Loyalists at the most decisive moment.

Prior to the landing on the planet, Fulgrim and the Emperor's Children built fortifications and trenches, intended to protect the Traitor Legions from Loyalist orbital bombardment. The initial naval operations seemed to go well for the Loyalist side. The Imperial Navy managed to make orbit over Isstvan V and the Loyalist Legions proceeded with their planetary deployment. Thousands of Drop Pods and Stormbirds were deployed for the drop.

The first wave was under the overall command of the Primarch Ferrus Manus and besides his own Legion, the Iron Hands, the Loyalist forces included the Salamanders led by Vulkan, and the Raven Guard under the command of their Primarch Corax.

Vulkan's Legion assaulted the left flank of the Traitors, Ferrus Manus, Gabriel Santar, and 10 full companies of elite Terminators (called Morlock Terminators by the Iron Hands) went straight to the middle of the enemy lines, and Corax's Legion hit the right flank of the enemy position. The odds were considered equal; 30,000 Traitor Marines against 40,000 Loyalists.

Morlock Terminator Squad

Two Iron Hands Morlocks Terminators, fighting valiantly during the Drop Site Massacre on Isstvan V

Horus was aware of the location of the Loyalists' chosen drop site and his troops fell upon the surprised Loyalist Legions. Ferrus Manus engaged Fulgrim in a ferocious duel, but ultimately the Primarch of the Iron Hands failed and was slain. The Loyalists retreated towards the apparent safety of the reserve force's lines.

Yet the Space Marine Legions of the reserve were no longer loyal to the Emperor, having already secretly sworn themselves to Chaos and the Warmaster. The Night Lords of Konrad Curze, the Iron Warriors of Perturabo, the Word Bearers of Lorgar, and the Alpha Legion of Alpharius fell upon their unsuspecting Loyalist brothers unexpectedly and the ensuing slaughter is widely known as the Drop Site Massacre of Isstvan V.

After the battle, the head of Ferrus Manus was delivered by Fulgrim to Horus as a trophy of the battle. Only a few Loyalist Space Marines, bearing the gene-seed of their fallen brothers and carrying the critically wounded Primarch Corax, managed to escape.

A Loyalist primarch had fallen in battle, and another was severely wounded. Still a third Loyalist primarch, Vulkan, went missing in action after a tactical nuclear assault on his section of the battlefield and he was presumed dead.

How he ultimately managed to survive and to escape the Traitor Legions remained largely unknown to the wider Imperium, though the fact that he was a Perpetual and thus capable of resurrection from almost any form of death now explains his unexpected survival.

Istvaan V Massacre

The Iron Hands and Emperor's Children Legions clash on the black sands of the Urgall Depression.

The outcome of the Drop Site Massacre was disastrous news for the Emperor and the Imperium. Due to the developments at the world of Prospero the forces of the rebellion against the Emperor were further strengthened by Magnus the Red and his Legion, the Thousand Sons, who had become the unwitting servants of Tzeentch and had ultimately chosen to serve the Changer of Ways to protect their precious knowledge of sorcery.

As the Heresy began in earnest, Horus possessed nine Space Marine Legions and had all but destroyed three of the remaining nine Loyalist ones. The path to Terra was wide open, and the decisive Siege of Terra would follow after seven more standard years of blood and terror as the Traitor Legions penetrated to the very heart of the Imperium of Man.

Operation Carthage

The Desert Lions Space Marines were assigned to retake Isstvan V from the forces of Chaos after the end of the Horus Heresy at some point in the early 31st Millennium during a campaign known as Operation Carthage alongside a cohort of Battle-Automata from the Adeptus Mechanicus' Legio Cybernetica.

The Chapter had been tasked with the difficult job of reducing the enemies' planetary fortresses. To deal with the problem, the Desert Lions programmed the Legio Cybernetica robots to assault the Chaotic citadels in a seemingly mindless fashion.

Though the robots were easily defeated and almost wiped out by the defenders, the automatons also succeeded in providing invaluable intelligence to the Astartes on the nature of the Chaos forces' defences, including their firing plans and the various blind spots in the fortresses.

With this information, when the Desert Lions launched their own assault upon the fortresses, they were able to quickly reduce the defenders and bring the world back under Imperial control. Only 7 Desert Lions Space Marines were lost during the assault. In gratitude, the Desert Lions inducted all of the surviving robots of the Legio Cybernetica cohort into the Chapter as honourary Battle-Brothers.



Warhammer 40,000 Grim Dark Lore Part 27 – Massacre

Canon Conflict

The system/planet's name is alternatively spelled Isstvan.

An older version of this battle reports that initially the Emperor was quite reluctant to crush the revolt, as the reasons for the actions of the Warmaster were unknown.

In this version of the story, Fulgrim initially tried to reason and negotiate with his brother Primarch Horus only to be seduced by the power of Slaanesh.


  • Codex: Chaos Space Marines (3rd Edition, Revised Codex), pg. 5
  • Horus Heresy: Collected Visions
  • The Horus Heresy - Book Two: Massacre (Forge World Series) by Alan Bligh, pg. 26 (sidebar)
  • Index Astartes III
  • Index Astartes IV
  • Realm of Chaos: Slaves to Darkness (2nd Edition)
  • Renegades, pg. 61, by Andy Chambers, Jervis Johnson and Rick Priestley
  • Raven's Flight (Audiobook) by Gav Thorpe
  • Fulgrim (Novel) by Graham McNeill
  • The First Heretic (Novel) by Aaron Dembski-Bowden
  • Aurelian (Novella) by Aaron Dembski-Bowden
  • The Outcast Dead (Novel) by Graham McNeill
  • Deliverance Lost (Novel) by Gav Thorpe
  • Warhammer 40,000 Compendium (1st Edition), pp. 97, 105
  • White Dwarf 104 (UK), "Chapter Approved: Imperial Robots" by Mike Brunton
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Warhammer 40,000 Overview Grim Dark Lore Teaser TrailerPart 1: ExodusPart 2: The Golden AgePart 3: Old NightPart 4: Rise of the EmperorPart 5: UnityPart 6: Lords of MarsPart 7: The Machine GodPart 8: ImperiumPart 9: The Fall of the AeldariPart 10: Gods and DaemonsPart 11: Great Crusade BeginsPart 12: The Son of StrifePart 13: Lost and FoundPart 14: A Thousand SonsPart 15: Bearer of the WordPart 16: The Perfect CityPart 17: Triumph at UllanorPart 18: Return to TerraPart 19: Council of NikaeaPart 20: Serpent in the GardenPart 21: Horus FallingPart 22: TraitorsPart 23: Folly of MagnusPart 24: Dark GambitsPart 25: HeresyPart 26: Flight of the EisensteinPart 27: MassacrePart 28: Requiem for a DreamPart 29: The SiegePart 30: Imperium InvictusPart 31: The Age of RebirthPart 32: The Rise of AbaddonPart 33: Saints and BeastsPart 34: InterregnumPart 35: Age of ApostasyPart 36: The Great DevourerPart 37: The Time of EndingPart 38: The 13th Black CrusadePart 39: ResurrectionPart 40: Indomitus