Horus Heresy - Battle of Calth: While Horus made his plans for what would become known as the infamous Drop Site Massacre on Isstvan V, the Warmaster sent word to the XVII 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 XIII Legion, the Ultramarines, who had once humiliated the Word Bearers by destroying their city of Monarchia on the world of Khur at the Emperor's orders during 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 XIII Legion.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 XIII and the XVII 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 XIII 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 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 XIII Legion had destroyed one of the Word Bearers' greatest achievements, the sacred city of Monarchia, four decades earlier.Horus ordered the majority of the XVII 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 XVII 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 XIII 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.