Calth - Battle of Calth: It is unlikely that the true architect of the Calth atrocities will ever be known however, 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 Istvvan, 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.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 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.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.Though initially caught off guard by the treacherous surprise assault, the Ultramarines successfully broke the Word Bearers' attack after a viciously-fought siege action, though at the cost of terrible casualties and the complete destruction of Calth's atmosphere and once-verdant biosphere. As a result of the devastation wrought by the Word Bearers during the Calth Atrocity, future generations of Calth's people were required to live deep underground in massive subterranean arcologies to escape their world's radiation-scorched, airless surface. While both the Ultramarines and their Primarch Roboute Guilliman survived the Word Bearers' assault, the campaign successfully prevented the Ultramarines from participating in the Battle of Terra as Horus had planned.