Battle of Calth - The Underworld War: 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 XIIIth 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 XIIIth 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 led 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 as its first Lord Commander, 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.