Battle of Calth - Striking a Blow for Horus: During the opening days of the terrible conflict that would become known as the Horus Heresy in later years, the Warmaster Horus launched his treacherous attack on the world of Istvaan III against the Loyalist elements within four of the Space Marine Legions that turned to Chaos, including the Sons of Horus, the Emperor's Children, the World Eaters and the Death Guard. It is believed by Imperial savants that the Loyalists made up approximately one-third of the combatants at Istvaan III. Having successfully purged the remaining Loyalists from the Traitor Legions, Horus plotted the next stages of his insurrection against his once-beloved father, the Emperor of Mankind. When word reached Terra of the Warmaster’s treachery at Istvaan III - a result of the flight of the frigate Eisenstein under the command of Captain Nathaniel Garro and a small group of Loyalist Space Marines from the Death Guard Legion - the Emperor ordered the deployment of seven Loyalist Astartes Legions to the Istvaan System to bring Horus to account for his actions.While Horus made his plans for what would become known as the infamous Drop Site Massacre on Istvaan V, the Warmaster sent word to the XVIIth Legion's Primarch Lorgar that the time had come for his Astartes, the Word Bearers, to strike against the Imperium. The Warmaster was keenly aware of the bitter hatred that Lorgar had for his Primarch brother Roboute Guilliman and his XIIIth Legion, the Ultramarines. The Emperor had grown impatient with the Word Bearers progress in the Great Crusade as they began to delay longer and longer at their conquests, building temples and fanes in His honour, many among the Officio Militaris and the Emperor's inner council began to question their priorities. In 963.M30 the Emperor sent Malcador the Sigillite, first among his councillors, and Roboute Guilliman of the Ultramarines to correct Lorgar's ways. At His order, the Word Bearers were publicly censured and the city of Monarchia on the distant world of Khur, along with its false temples, was destroyed by the warriors of Ultramar as an object lesson in the folly of false religion. The entirety of the XVIIth Legion was then forced to kneel in the ashes of their devotion while the Ultramarines stood over them in judgement, that which they had seen as worthy devotion now labelled treasonous. The Ultramarines had taken no pleasure in this act, which was intended to teach Lorgar and his Astartes to adhere to the atheistic doctrines of the Imperial Truth rather than spread the false belief that the Emperor was divine to all the worlds that they conquered. Yet Lorgar and the Word Bearers had never forgiven the Ultramarines for this action and they longed for vengeance against the XIIIth Legion.It is unlikely that the true architect of the Calth atrocities will ever be known, but 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 Istvaan, in the year 005.M31, with a series of orders issued under the seal of the Warmaster dispatching 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 Ultramarines would muster alongside the Word Bearers at Calth. Horus told Lorgar that he had fed Guilliman false intelligence in regard to a possible threat within the Veridian System in the Segmentum Tempestus, far to the galactic south of Terra. This supposed threat stemmed from the Orks of the Ghaslakh Empire. Horus had ordered the XIIIth and the XVIIth Legions to muster and meet at the world of Calth in the Ultramarines' Realm of Ultramar, in order to conduct a massive joint campaign of extermination against the Ghaslakh xenohold, a common mission for the Astartes during the final days of the Great Crusade.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 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 Istvaan, 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.It would be at Calth that Lorgar would launch a surprise attack on the Ultramarines whilst they were gathered for the campaign against the Orks of Ghaslakh. The XIIIth Legion would be caught completely unaware while the Word Bearers would use the advantage of surprise to completely annihilate their hated rivals. The assault at Calth would also allow the Word Bearers to reveal that they, too, now served the Ruinous Powers. Calth was not chosen as the site of the confrontation between the Word Bearers and the Ultramarines by chance, for the Word Bearers intended to destroy one of the jewels in the Ultramarines' realm of Ultramar (then known as the Ultramar Coalition), just as the XIIIth Legion had destroyed one of the Word Bearers' greatest achievements, the sacred city of Monarchia, four decades earlier.Horus ordered the majority of the XVIIth Legion to Ultramar, and the dark powers of the Warp gave them sure and swift passage across the increasingly restless Immaterium.As the Word Bearers entered Ultramar space, Lorgar prepared his Legion for the inevitable slaughter that would follow. Command of the main assault force was given to Kor Phaeron, the First Captain of the XVIIth Legion and one of Lorgar's most favoured champions. Calth was to be Kor Phaeron's operation to execute, far more than it was Lorgar's. Kor Phaeron had planned the assault of Calth for his Primarch meticulously, and executed it with the aid of the Dark Apostle Erebus. The punishment and annihilation of the XIIIth Legion was the campaign's principal aim; the humiliation and execution of Lorgar's hated rival Roboute Guilliman was a secondary objective. But for Lorgar, the assault would also mark his first opportunity to gain true favour in the eyes of the Dark Gods he now served; to prove to them that he had earned his place as their Chosen One.The first phase of the Word Bearers' attack plan involved the capture of a vessel of the Ultramarines' fleet. The act which heralded the Battle of Calth went unseen and undetected at the time, by human eyes at least, for it was performed in the silent outer reaches of the Veridian System. That act was the taking of an aged fleet auxiliary tender by the name of the Campanile, and no report was made of this event and none survived it. Whether she was captured by some warp-tainted psyker fated to die as an act of ritual sacrifice, or some warp entity breached her hull, slaughtered the entire crew and took total control of the vessel's systems matters not. The use to which she was employed betrays the genius of a Primarch allied in unspeakable compact with powers then only just stirring and known to few others than he. Soon after the Campanile fell silent, an event barely noted by the massively over-taxed Veridian System Control, the first of the Word Bearers fleet broke Warp to complete the conjunction of the two Legions. The Traitors intended to use the captured vessel to strike a crippling surprise blow against the Ultramarines' massed fleet that would mark the start of their vicious assault.