The Ruinstorm was a massive Warp Storm that blocked off large portions of the Milky Way Galaxy to interstellar travel and communications during the Horus Heresy. The massive Warp disturbance had been engineered by Lorgar, the Primarch of the Word Bearers Traitor Legion, to isolate the Loyalist forces of the Emperor in different places across the galaxy so that they could not cooperate in defence of the Imperium of Man against the Warmaster Horus' drive on Terra.
The Ruinstorm was unleashed by the Word Bearers First Chaplain Erebus during the Battle of Calth between the Word Bearers and the Loyalist Ultramarines Legion. The Ruinstorm grew greater in size and even more potent in blocking off portions of the galaxy in the years after the destruction of Calth, its growth fuelled by the psychic emanations released into the Immaterium by the misery of Lorgar's Shadow Crusade.
While the Ruinstorm remained active, the Warp in many regions of the galaxy often proved incapable of being navigated by Loyalist forces. However, the Traitor Legions and their allies were able to make their way through the Ruinstorm by using secret passages through the Immaterium revealed to Lorgar by the Ruinous Powers.
After the Second Battle of Davin in ca. 011.M31, the strength of the Ruinstorm began to wane to the point that Rogal Dorn, the Primarch of the Imperial Fists and the Praetorian of Terra, came to believe that the Ultramarines would be able to reach Terra in time to help defend it from assault by the Traitor forces of Horus. In truth, Horus had foreseen the weakening of the Ruinstorm and had erected a blockade to the Loyalists' passage through it with a portion of his fleet.
Instead, the Ultramarines and Dark Angels Legions distracted the forces of the Traitor blockade while Sanguinius and the entire Blood Angels Legion made directly for Terra as the Angel has foreseen in his visions of the future, where they would arrive in time to play a pivotal role in the defeat of Horus.
In the wake of the defeat of Horus and the Forces of Chaos at the end of the Horus Heresy, the Ruinstorm would fully dissipate back into the æther as the power of the Chaos Gods waned with the defeat of their champion by the Emperor.
Mustering at Calth on orders issued by the Warmaster Horus before the massacre of Loyalist forces at Istvaan III, the Ultramarines and Word Bearers prepared for a campaign against the Orks of the Ghaslakh xeno-hold. The Ultramarines were unaware of the Word Bearers' true allegiance and utterly unprepared for betrayal, and suffered horrific losses during the subsequent Calth Atrocity. The Traitors' real objective was the murder of the Veridia star as an offering to the Ruinous Powers that would bring about the Ruinstorm, a Warp tempest so large, it would sever the galaxy in half and render all long-range Warp travel unpredictably dangerous, and trans-luminal communication all but impossible. What would later be known as the phase of the great conflict called the "Age of Darkness" had begun.
Lorgar's invocation of the Ruinstorm cut the galaxy asunder, and so most Legiones Astartes forces not operating under the personal command of their Primarch were often required to act under their own cognisance for extended periods, in many cases for the entirety of the Horus Heresy and long after. The Ruinstorm did not serve only to stymie the coordination of far-flung armies, but had a more unsettling effect born of the Dark Gods underpinning the entire conflict. Even though it existed primarily in the other-dimension of Warpspace, the Ruinstorm bled through into realspace in many regions, the raw stuff of the Immaterium tainting the void itself with coiling æetheric tendrils and kaleidoscopes of unseen colours. From the surface of many worlds, the stars appeared to have been swallowed by the void or the night skies were transformed into a canvas upon which were rendered scenes from the hells of ancient superstitious texts.
On the worst afflicted worlds, nightmares made flesh walked the lands, the veil between reality and the Immaterium stretched beyond the breaking point and the infernal denizens first encountered by the Ultramarines at Calth -- named by some "daemons" -- came spewing forth. Thus, in many cases, it was the smaller, detached Legiones Astartes forces that first encountered such abominations as they fought far from the key war zones, prosecuting isolated wars against creatures that defied all sanity and had no place in the secular Imperium the Great Crusade had founded upon the bedrock of Imperial Truth. By 010.M31, the Imperium of Man that had been stood no more. The fires of the Horus Heresy were consuming hundreds of star systems in all-out war, while innumerable conflicts burned in the darkness far beyond the central war zones.
Envoys of Terra
With the Ruinstorm surging across the galaxy, the War Council of Terra sought by any means to gauge the true scope of the unfolding calamity. Blinded as it was, the Council was desperate for other ways to measure the disposition of Horus' hosts and the allegiances of planetary rulers. Astra-telepathic communication was by that point all but impossible, the Warp a howling cacophony that either drowned out their messages or corrupted their content in insidious ways. The only means of assaying the status and allegiance of the scattered worlds of the Imperium would be to despatch envoys and agents to do so in person, even though the Council knew that such a hazardous mission would be the deaths of countless loyal and devoted servants of Terra.
A Kingdom Divided
Entire cohorts of Imperial historators and savants have attempted to piece together a coherent picture of the status and deployment of the major Legiones Astartes forces in this period when the Ruinstorm raged unabated. In all likelihood such a picture is impossible to compose in entirety, and so their knowledge must remain incomplete. For every world that burned in planet-wide conflagration, innumerable smaller scale, but equally bitter wars were fought, often far away from any war zone that might be considered a front line.
At the opening of the Horus Heresy, many of the Legions had been operating as large bodies under the direct command of their Primarch, but even then, many smaller contingents had been seconded to service elsewhere. As the Age of Darkness ground on, many of these mighty hosts became fragmented and while individual Primarchs often remained in direct command of the core of their Legions, ever more sub-commands were despatched across the galaxy in pursuit of either Traitor or Loyalist goals.
As the Ruinstorm cut yet more regions of the galaxy off from the greater Imperium, many such detachments were turned by circumstance into independent commands in all but name, their leaders vested with what amounted to total authority to conduct their own wars in whatever manner they deemed most appropriate. Across the galaxy, independent Legiones Astartes forces varying in scale from sub-company to battalion or chapter conducted countless high intensity operations. Many were surgical strikes against targets of vital strategic worth. Many operations were specifically targeted attacks against key enemy leaders or individuals holding the balance of power, and many of these amounted to little more than callous murders and brutal assassinations. Far-ranging Legiones Astartes Kill-teams or larger strike forces were capable of reaching and striking against targets more conventional Astartes forces could not engage, and so no commander or high administrator was guaranteed safety from the executioner's axe.
Some Loyalist forces pledged themselves to defend individual worlds against the depredations of the Traitors, in particular if they found themselves stranded by the coming of the Ruinstorm and unable to penetrate its depths. History can never make a full accounting of the number of such actions fought by stoic and self-sacrificing Loyalists far from the main war zones; indeed, only they and possibly those they protected will ever know of the sacrifices made. Other Legiones Astartes ranged for thousands of light years across the galaxy, braving the perils of the storm-wracked Warp in an effort to engage the enemy wherever they were to be found.
The bulk of the I Legion was engaged against the Night Lords for much of the period, and in the aftermath of the Lion's victory over the Traitor Primarch at Tsagualsa despatched numerous strike forces across the eastern galaxy to hunt down the splintered foe. Many did not return, pursuing their own missions across the Ultima Segmentum and beyond. Some of these detached forces encountered other similarly isolated or orphaned Legiones Astartes groups, finding common cause with some and fighting others to the death, the Ruinstorm making it impossible for them to return home or receive new orders.
Among the forces loyal to the Emperor that were isolated by the Ruinstorm was the Blood Angels Legion, the majority of whose Astartes were marooned in the Signus Cluster and cut off from communication with their Legion homeworld of Baal. At the opening of the Horus Heresy, the fate of the Blood Angels was unknown and this would remain the case until the Legion's return to contact with the broader Imperium in 010.M31.
The Blood Angels had been ordered to muster at the Nartaba System and from there to deploy to Signus Prime to crush a major recidivist uprising. Several thousand Astartes never arrived there however, some serving as Lamentii honour guardians watching over silent battlefields wherever the Legion had shed its blood while others simply never reached Nartaba, their vessels cast far and wide upon the tides of the Warp when the Ruinstorm struck.
It was only later that news of their survival began to filter out and then only very slowly thanks to the Ruinstorm. For a period of almost five standard years, many of those lost and scattered Blood Angels forces who survived their mis-jumps to Nartaba had every reason to believe themselves orphans and the last remaining survivors of their Legion.
Another Space Marine Legion greatly affected by the Ruinstorm were the Ultramarines. The bulk of the XIII Legion had mustered at Calth and for most of 007.M31 and in 008.M31 they were engaged in the bitter war to repulse the so-called Shadow Crusade of the Primarchs Angron and Lorgar. Nevertheless, Legion detachments were stationed elsewhere, on lynchpin worlds such as Molech and serving in the Conquestor fleets of Rogue Trader Militants. There are likely to have been Ultramarines units that had not arrived at Calth when the Word Bearers' betrayal struck, and such units were likely to have operated as independent, autonomous forces until they re-established contact with their chain of command.
The Ruinstorm eventually grew so strong and covered so much of the galaxy that Primarch Roboute Guilliman of the Ultramarines feared that the Throneworld of Terra itself was lost to the Loyalists. To maintain the power of the Imperium and the ideals of the Emperor of Mankind in the region now cut off from Terra's control, Guilliman proceeded to create his own auxiliary Imperial state, the Imperium Secundus.
Only the use of the pre-Imperial xenos biotechnological artefact called the Pharos allowed the Loyalists of the Imperium Secundus to navigate through the great Warp Storm as if using a much weaker version of the later Astronomican and regroup on the Ultramarines' homeworld of Macragge.
Age of Darkness
Between the eruption of the Ruinstorm across the galaxy in early 007.M31 and Roboute Guilliman's establishment of the Imperium Secundus in 010.M31, civil war erupted across the entire galaxy. While most of the Space Marine Legions, whether Traitor or Loyalist, operated as mighty armies led in person by their Primarch, every one of them had smaller forces detached to serve myriad roles across the Imperium.
With the coming of the Ruinstorm, many of these detached forces were lost, cut off, isolated or simply never heard from again. In some cases, Great Crusade detachments returning from the outer void to find the Imperium in flames had no way of knowing the fate of their brethren, and were forced by circumstance to forge their own fates and to determine their own allegiance in a galaxy of blood, war and chaos.
Second Battle of Davin
During the Horus Heresy, the Space Marine Legions trapped behind the Ruinstorm -- the Ultramarines, Blood Angels and Dark Angels -- had sought to breach the great Warp Storm so they could make their way to Terra and aid in its defence from the assault by the Warmaster Horus' Traitor Legions.
The three Legions successfully defeated a daemonic assault at the world of Davin during the Second Battle of Davin and then used Cyclonic Torpedoes upon that cursed planet to destroy it. In the place of where Davin once was, a breach in the Ruinstorm had become visible.
The opened path led to Terra, but upon further study it became apparent that somehow Horus had foreseen this route and a large blockade of Traitor warships had been erected to stop any passage.
Roboute Guilliman and Lion El'Jonson agreed to use their forces to distract the Traitor vessels while Sanguinius and the Blood Angels made directly for Terra. The arrival of the Blood Angels on Terra proved crucial to the Imperium's victory during the Siege of Terra, though it cost the life of Sanguinius as he had foreseen when he was slain by Horus aboard the Vengeful Spirit. In addition, the destruction of Davin proved to weaken the Ruinstorm, which slowly began to dissipate after the battle, eventually evaporating completely by the end of the Heresy.
- The Horus Heresy - Book Five: Tempest by Alan Bligh, pp. 33, 37, 41, 69, 141, 145, 154, 158, 172
- The Horus Heresy - Book Six: Retribution by Alan Bligh, pp. 13-14, 16, 20, 29-30, 56, 84, 88, 152, 200
- Visions of Heresy - War, Darkness, Treachery and Death (Omnibus) by Alan Merrett, pp. 282-283
- Lost Sons (Short Story) by James Swallow
- Fear to Tread (Novel) by James Swallow
- Know No Fear (Novel) by Dan Abnett
- The Unremembered Empire (Novel) by Dan Abnett
- Ruinstorm (Novel) by David Annandale, Chs. 17-Epilogue