Macragge's Honour - 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 Isstvan III against the Loyalist elements within four of the Space Marine Legions under his command, including the Sons of Horus, the Emperor's Children, the World Eaters and the Death Guard. The Ultramarines remained ignorant of this fact.While Horus made his plans for what would become the infamous Drop Site Massacre on Isstvan 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, who had once humiliated the Word Bearers by destroying their "Perfect City" of Monarchia on the world of Khur on the Emperor's direct orders during the early years of the Great Crusade.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.Horus told Lorgar that he had fed Guilliman false intelligence in regard to a possible threat within 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.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 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 Ultramarran 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 Word Bearers waylaid the fleet tender Campanile which was at the outer rim of the Viridian System which was home to Calth, boarded it and seized control. The Traitors intended to use the captured vessel to strike a crippling surprise blow against the Ultramarines' massed fleet in orbit of Calth that would mark the start of their vicious assault.