Ultramar - Battle of Calth: It is unlikely that the true architect of the Calth Atrocity will ever be known, and weighed against the innumerable sins committed by both Lorgar and Horus, the attribution of this one offence is inconsequential. What is known is that the earliest stages of the assault on Ultramar were laid down long before Horus arrived at the fateful worlds of Isstvan III, in the year 005.M31, with a series of orders issued under the seal of the Warmaster despatching a number of Legions to campaigns in the furthest reaches of the Imperium.Of these, the Blood Angels were sent forth in their entirety to Signus, the Dark Angels to Tsagualsa and the Ultramarines received orders to muster alongside the Word Bearers at Calth, both Legions to be deployed against the Ork hold of Ghaslakh. Gathering at Saturn those of his Legion who had been embarked on crusades in distant parts of the galaxy, Roboute Guilliman would depart the Sol System mere solar months before news of Horus' rebellion reached the Emperor's ears.The turbulent state of the Empyrean in those years would see the Ultramarines' main strength journey to Calth by a winding and obtuse trail which would also cloak them from all attempts by Terra to recall them or forewarn them of Horus' actions. The Word Bearers, delayed by the slaughter at Isstvan, would not arrive at Calth until the majority of the XIIIth Legion had already gathered, travelling a path of blood and ashes of their creation.Though the Ultramarines would eventually prove victorious, they would not be afforded the luxury of reaping the fruits of their victory. In one final callous act, the Word Bearers somehow, and through inexplicable means, poisoned the solar winds emanating from Calth's Veridia star. The surface of Calth, its atmosphere already grievously injured by the Word Bearers' bombardment, rapidly destabilised to the point that it soon became impossible for unprotected mortals to walk upon it. The majority of the population was evacuated into the vast subterranean arcologies in an effort to provide some shelter, however tenuous, from the fell light of the Veridia star.The second crisis brought to the primarch's attention was at that time only partially understood, but already astrotelepathic messages were relaying desperate pleas for aid from all over the Five Hundred Worlds of Ultramar. The Traitors' assault on Calth was not an isolated event; other attacks were being reported across dozens of other star systems. In a moment of stark clarity, Guilliman saw what the Word Bearers had intended at Calth -- they had planned the extinction of the Ultramarines so that the Traitors could reave unopposed across those stars under his protection.In this they had failed, for while the Ultramarines had suffered unprecedented losses, the Legion had rallied and was yet a formidable force. Guilliman's vengeance would be terrible indeed, and well-earned. But here the primarch's counsellors set their final piece of ill news before him. The Warp, which had been turbulent and capricious for the past standard year, now rose towards a storm of unprecedented fury. Worse, it closed in around the Veridian System and any vessels that did not leave within a few solar hours would be stranded there for standard years.The losses inflicted upon the Ultramarines' fleet assets at Calth were crippling and hamstrung any effort to prosecute a war beyond the bounds of Ultramar. Such was the Ultramarines' desperation, missions to salvage the hulks drifting in orbit above Calth were quickly authorised despite the death toll such missions exacted in the deadly radiation of the altered Veridia star.Eventually, Guilliman took his leave of the once-bountiful world of Calth, the would-be jewel in the crown of Ultramar which had stood for so much that now could never be realised. Before the primarch ordered the fleet to depart, the Warp closing in all about it, he swore to return to Calth and to deliver it from the Traitors. It would be many standard years and many battles, across five hundred worlds and more, before that promise could be delivered upon. But it would be delivered upon, for Roboute Guilliman, primarch of the XIIIth Legion and Master of the Five Hundred Worlds of Ultramar, had sworn it. Ultramar would endure.