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The '''Battle of Isstvan III''' (also referred to as the '''Isstvan III Atrocity'''{{Fn|1c}}{{Fn|1d}}) was the start of the [[Horus Heresy]] and was fought between the [[Adeptus Astartes]] loyal to the [[Emperor of Mankind]] and those traitors that swore fealty to [[Imperial Guard Hierarchy#Warmaster|Warmaster]] [[Horus]].{{Fn|1a}}

Over a year before the battle, [[Warmaster]] [[Horus Lupercal]] had [[Battle of Davin's Moon|fallen]] on the moon of [[Davin]] thanks to a conspiracy by the [[Word Bearers]].{{Fn|3}} He immediately set about constructing a plan to overthrow the [[Emperor]]. Still posing as the loyal [[Warmaster]] of the Throne of [[Terra]], Horus weaved a web of deceit and betrayal in preparation for open rebellion. This included sending Primarchs deemed likely to remain loyal such as [[Lion El'Jonson]], [[Roboute Guilliman]], and [[Sanguinius]] to the fringes of the Imperium to ensure they could not interfere with what was about to transpire. Meanwhile, he assembled those Legions whose Primarchs he had managed to turn to his cause: [[Fulgrim]] and the [[Emperor's Children]], [[Angron]] and the [[World Eaters]], and [[Mortarion]] of the [[Death Guard]], planning to use the coming battle as a way to purge this force of remaining loyalist elements.{{Fn|1a}}

So it was that some 200,000 Astartes assembled in the [[Isstvan System]], ostensibly to counter an anti-Imperial rebellion on [[Isstvan III]]. The Death Guard [[Space Marine Legion|Legion]] had received a distress signal from Isstvan III which stated that the colony had turned away from the [[Imperial Truth]] and that the former [[Planetary Governor|governor]], [[Vardus Praal]], was leading the rebellion. The assembled Imperial expedition was tasked with eliminating this threat. While Mortarion and Angron were able to rendezvous with Horus at Isstvan III, Fulgrim was dispatched to potentially turn [[Ferrus Manus]] to the traitor cause and the Emperor's Children at Isstvan were commanded by [[Lord Commander]] [[Eidolon]].{{Fn|1a}}

Initial skirmishes were fought on the outer system world of [[Isstvan Extremis]] where the Emperor's Children and Death Guard had managed to eliminate the enemy outpost. With the enemy forces blinded, Warmaster Horus brought about the Legions to hammer the rebel forces.{{Fn|1a}}{{Fn|1b}}

At the Precentor Palace, the Emperor's Children advanced room by room at significant cost thanks to the presence of many Warsingers. There were also large numbers of a previously unencountered type of rebel elite. These warriors were surgically modified and mutilated, clad in black vitrified armor, and wielding strange weapons that spat killing blasts of sound or darts of liquid metal. However they eventually were able to fight their way to the throneroom, where Captain [[Lucius (Emperor's Children)|Lucius]] slew the heavily augmented Vardus Praal personally.{{Fn|1h}}{{Fn|2d}} Simultaneously deep within the Siren's Hold Tomb Spires the Sons of Horus under the command of [[Garviel Loken]] fought into a strange corpse-filled shrine and slew many Warsingers before destroying the complex. These two acts managed to dispel the strange cacophony that held sway over the Choral City, and within hours the rebellion was smashed. Quickly securing their remaining objectives, the victorious Space Marines celebrated their hard-won victory, unaware that disaster was about to strike.{{Fn|1h}}

With those squads and their leaders that were known to be completely loyal to the Emperor on the planet, the Warmaster finally decided to initiate his plan. All communication with the orbiting fleet ceased and the Titans of the Legio Mortis went silent as they sealed themselves against biological threats.{{Fn|1h}} The warships ''[[Vengeful Spirit]]'', ''[[Firebird (Battleship)|Firebird]]'', ''[[Andronius]]'', ''[[Killing Star]]'', ''[[Indomitable Will]]'', ''[[Gauntlet of Spite]]'', ''[[Warchild]]'', ''[[Eisenstein]]''{{Fn|4a}}, and ''[[Conqueror (Ship)|Conqueror]]'' all lowered into low orbit and launched salvos of [[Virus Bomb]]s upon Isstvan III's surface.{{Fn|1h}} The assembled [[Remembrancer]] Order was present on the ''Vengeful Spirit'' while this happened. After Horus declared his intentions of overthrowing the Emperor, his Astartes went about exterminating the Remembrancers.{{Fn|2a}}

While most of the Astartes had survived in their shelters, a shock fell over them. Never before had such a terrible betrayal befallen the Legiones Astartes, and worse still their own Primarchs had disowned them. This shock soon turned into righteous defiance, and the remaining survivors pledged to fight to the last and not go down silently.{{Fn|1b}} But before Horus could formulate a new plan, the maddened [[Angron]] unexpectedly descended from the ''Conqueror'' to Isstvan III with 50 companies of frenzied World Eaters, intent on massacring the loyalists. The recklessness and insubordination infuriated Horus, who no longer could simply annihilate Isstvan from orbit without eliminating a key commander for his coming wider rebellion.{{Fn|1b}}

Seeing that they were betrayed, Captains [[Garviel Loken]] and [[Tarik Torgaddon]] of the Sons declared to the surviving Astartes of the XVI Legion that they were once again the Luna Wolves and marched to Praal's stronghold, where they dug in with the surviving members of the Emperor's Children under Saul Tarvitz and [[Lord Commander]] [[Abdemon]]{{Fn|1j}} and combated against the traitor forces.{{Fn|2e}} The loyalist World Eaters fell under the command of Captain [[Ehrlen]] and [[Centurion (Rank)|Centurion]] [[Shabran Darr]]{{Fn|1k}} while the Death Guard survivors were led by [[Crysos Morturg]]. The loyalists pledged to make their traitorous brothers pay for every inch of ground.{{Fn|1h}}

The first cracks in the loyalist defense came from the Death Guard in the northwest of the Choral City, who were faced with the most desperate situation as both Angron and Mortarion each advanced on their position. Already hammered by the vicious frontal assault of the World Eaters, Mortarion landed his great [[Astartes Assault Ship|relic assault ship]] ''[[Omen]]'' and used its turrets to sweep the ground clear of trenches and pillboxes. In their wake came heavy [[Spartan Assault Tank|Spartan]]s, [[Fellblade]]s and [[Typhon Heavy Siege Tank|Typhon]] tanks with Mortarion personally leading them. One by one, the loyalist defensive bastions were subdued.{{Fn|1m}}

Nearly two months into the fight, the last of the great storms began to wane and the loyalist situation rapidly worsened. Tarvitz and his Emperor's Children still held the Precentor's Palace and the Luna Wolves still held the Siren's Hold, but in the northwest the loyal Death Guard were finally smashed by Mortarion and Angron's onslaught. In a confusing battle dubbed the "Veil of Grief", the legendary discipline of the Death Guard broke down as some surviving loyal elements within Mortarion's own armies turned their guns upon their traitorous brothers or the World Eaters. Traitor and Loyalist became lost in the maelstrom, with Death Guard Marshal [[Durak Rask]] being killed by [[Crysos Morturg]]. Mortarion himself is said to have been wounded in the battle, burned by plasma fire from one of his own [[Predator]] tanks before he and his [[Deathshroud]] hacked it to pieces. The respite the confusion brought allowed some of the surviving loyal Death Guard to withdraw. In the aftermath a spiteful Mortarion used the waning storms to land massive amounts of reinforcements and poured heavy armor and Titans into the area, annihilating what resistance they still encountered. The loyalist Death Guard did not give up however, and withdrew to subterranean bunker and catacomb networks to continue to fight on. Mortarion had expected this however, and brought in [[Vortex Weapon|Vortex charges]] and [[Hades Breaching Drill]]s to clear out these remaining bastions.{{Fn|1m}}

There were ultimately only handful of loyalist survivors, such as Loken,{{Fn|5}} Morturg and Decima,{{Fn|6}} and Rylanor, who unknown to all sides remained hidden on the planet for millennia after.{{Fn|7}}

While victorious, the battle had proven far more costly and time-consuming than Horus could have envisioned. Moreover, by this point [[Fulgrim]] had finally returned with troubling news. Ferrus Manus had refused to join their cause and it was only a matter of time before their treachery was discovered by Terra. Horus finally withdrew his forces from Isstvan III, leaving an utterly dead world behind. He immediately set about for the next phase of his reformulated plan, regaining the initiative with an [[Drop Site Massacre|even greater act of treachery]] at [[Isstvan V]].{{Fn|1n}}{{Fn|2b}}

*[[Battles of the Horus Heresy (List)]]

{{Cite}}

*1: [[The Horus Heresy Book One - Betrayal]]: