Horus Heresy - Calth Atrocity: Before Horus had made his break with the Emperor of Mankind public at Isstvan III, he had sought to lure away as many Loyalist Legions from Terra as possible. Horus had ordered Roboute Guilliman to lead an expeditionary force to the world of Calth in the Veridian System in the Realm of Ultramar to prepare for a campaign in the Eastern Fringes of the galaxy, where, Horus claimed, an Ork WAAAGH! was massing. Horus expected the Ultramarines to await the arrival of the Word Bearers who would join with the XIII Legion in prosecuting a campaign against the Ork menace. Unknown to Guilliman, XVII Legion had long before turned Traitor in service to the Chaos Gods, and its Primarch, Lorgar, gleefully accepted Horus' orders to close the trap on his Legion's long-hated rivals. The Word Bearers' sudden attack decimated Guilliman's Legion fleet, and the Ultramarines' ground troops quickly found themselves impossibly outnumbered by their former allies as the infamous Battle of Calth erupted. The Word Bearers slew their Loyalist foes in droves in the early stages of their surprise attack and pushed them back over huge stretches of territory. The Traitors rejoiced at the terrible blows they were inflicting upon the Legion that had once aided the Emperor in humiliating them upon the world of Khur decades before the start of the Heresy when they had been taken to task for repeated violations of the atheistic philosophy known as the Imperial Truth. Unknown to them, Guilliman's flagship, which had survived the initial Word Bearers' attack on the Ultramarines fleet, effected emergency repairs and regrouped with the other surviving Ultramarine starships in space. Having taken stock of his remaining forces, Guilliman sent an immediate astropathic distress call to Macragge.The Loyalist Marines on Calth, Ultramarines all, had been forced into a fighting retreat, but soon occupied fortified positions. Many Ultramarines had been born on Calth, and proved more resolute than the Word Bearers anticipated. In space, Guilliman's vessels began hit-and-run attacks on their over-confident enemy. Guilliman assessed his ground troops' positions and broadcast clear, concise orders to each pocket of defence, coordinating them into a cohesive force. One Ultramarine force led by Captain Ventanus led a breakout and retook Calth's Defence Laser silos, aiding the sorely-pressed Ultramarines fleet from the surface of Calth. Guilliman's depleted forces slowed the Word Bearers down long enough for the remainder of the Ultramarines Legion to arrive and rout the Traitor Marines from the system, though at a heavy cost. The Word Bearers turned Calth's own orbital defence platforms on the Veridian star, stripping away the outer layers of its photosphere and destabilising it, ultimately rendering the surface of Calth uninhabitable. At the same time, the Word Bearers had used the battle taking place on Calth to summon a massive Warp Storm called the Ruinstorm, that was intended to cut off Ultramar from the rest of the galaxy and prevent the Ultramarines from providing any reinforcements to Terra as Horus made his assault upon humanity's homeworld. The eruption of the Ruinstorm cut off Calth from the main body of the Ultramarines Legion and left the Astartes of the XIII Legion trapped on Calth locked in a brutal subterranean war with those Word Bearers units that had also been left behind when their Legion retreated from the Viridian System. Yet Roboute Guilliman and a large portion of his Legion had remained off-world as a result of the Word Bearers' devious assault upon the Ultramarines fleet. Bloodied but unbowed, the Ultramarines received the orders of Malcador the Sigillite, the Emperor's Regent, while he was in disposed pursuing the secret Imperial Webway Project, and prepared to meet the needs of the Imperium's defence against the Traitor Legions as best they could.The Word Bearers' use of Calth's orbital defence platforms against Calth’s sun destabilised it, tore away the outer layers of its photosphere and threatened to cause it to explode as a supernova. The Veridian System's star, its colour changing from a bright yellow to an angry blue as its internal composition shifted, immediately suffered a flare trauma, and shortly after unleashed massive solar flares that irradiated Calth with lethal levels of radiation and stripped away its once dense oxygen-nitrogen atmosphere. The surface of Calth was no longer a safe environment for human habitation or any form of organic life, nor was it possible to evacuate the planet's remaining population in time. Therefore, Captain Ventanus sent out a warning over the planetary Vox and data network to all citizens, soldiers of the Imperial Army, Astartes of the XIII Legion, and any other loyal servant of the Imperium, to move with all haste to the subterranean arcology or arcology system closest to them. The arcology systems offered sufficient protection for the remaining inhabitants of Calth and their defenders to survive the wave of solar death that was about to engulf their world. This message successfully reached many millions of people who successfully took shelter within Calth's underground warrens, which had originally been built to free up more land for agricultural cultivation. It would take several Terran years for the remnants of the Ultramarines fleet to return to the Veridian System after the Loyalist survivors rode out the storm. Unfortunately, the Forces of Chaos left on the planet also fled underground as well. Vetanus vowed that the Ultramarines and their allied Loyalist forces would continue to fight until every last Traitor on Calth had been exterminated. Thus began the phase of the conflict that would be remembered as the Underworld War.Yet, despite the Loyalists' last-minute victory and the survival of the Ultramarines Legion and its Primarch, the Forces of Chaos could consider their assault on Calth a success. The XIII Legion had been badly crippled and no longer presented a viable threat to Horus' plan to drive on Terra. Erebus had managed to complete his blasphemous ritual on Calth's surface, which summoned a Ruinstorm to the galaxy's Eastern Fringe -- a monstrous Warp Storm larger and more destructive than anything space-faring humanity had witnessed since the days of the Age of Strife. It would split the void asunder, dividing the galaxy in two and rendering vast tracts of the Imperium impassable for centuries.The Ruinstorm would also isolate and trap those Loyalist forces caught behind it like the Ultramarines, preventing them from coordinating their efforts and supporting one another as the Traitor Legions moved towards Terra. It would even prevent them from warning each other, for a time, of the Warmaster's betrayal and the civil war that had begun to consume the Imperium. The Ruinstorm would leave Terra alone in the void, infinitely vulnerable to the approaching shadow of Horus.Some of the warships destroyed during the Calth Atrocity would continue to circle the tortured Veridian star for over 100,000 years as frozen wrecks. They served as silent tombs for the dead, eternal funerary monuments to the greatest betrayal in human history.The Ultramarines learned another hard lesson at Calth. The Calth Atrocity represented the Loyalist Astartes' first sustained experience with fighting the Warp entities later known as daemons in realspace. The Ultramarines realised that their decision to accept the anti-psyker dictates of the Council of Nikaea had lead to their voluntary surrender of the one weapon that might have proved most potent against the horrors of the Warp. It was almost as if the Traitors had known what was coming, and had orchestrated events so that the Imperium would voluntarily cast aside the only practical weapon it possessed against the sorceries of the Empyrean just before it was needed most. After the Ultramarines' experience at Calth, Roboute Guilliman sought to petition the Emperor for the revocation of the Edicts of Nikaea and the reinstatement of the corps of Librarians within all of the Loyalist Space Marine Legions. When the Horus Heresy ended and Roboute Guilliman initiated the Second Founding and the Reformation of the Imperium, he would get his wish, and the Imperium would once more make use of psychic powers against the Forces of Chaos despite the inherent dangers.Though the XVII Legion had achieved a monumental victory of sorts, it was all a matter of perspective. Piercing the veil of the Warp, Lorgar had heard the whisperings of the Chaos Gods and had witnessed the truth for himself. Yes, Erebus had successfully conjured the Ruinstorm at Calth. But ultimately, Erebus and Kor Phaeron had failed to achieve their overall objectives: Roboute Guilliman was still alive, the Word Bearers had lost half the fleet at Calth to an Ultramarines counter-attack, and tens of thousands of Word Beaers, including the Gal Vorbak and mortal servants, had been abandoned to a useless subterranean war beneath Calth's irradiated surface while the two Word Bearers commanders had fled. This meant that the Word Bearers left behind were left to die, never to be reinforced. Never to be recovered. All those Gal Vorbak who had spent months of their lives fasting, praying, scarring their flesh in preparation for a chance to taste the Divine Blood, they had simply been lost for no real gain. Though Lorgar was somewhat displeased, Erebus had more or less achieved the base level of success required of him; the Ruinstorm had been conjured and the rogue elements of the Word Bearers had been culled. Now it was time for Lorgar to further his own plans and complete the campaign against the Ultramarines he had come to call his Shadow Crusade.