The Carthaxian Diocese oversees rule on five planets, the greatest of which is Mellennor in the Al-Gheri sub-sector. Prior to 879.M41, there was another Cardinal World under their stewardship, misty Kayan of the Antonine Cluster.
Claimed by the Ministorum in the 33rd millennium, Kayan had previously been a feudal world, home to a number of warring fiefdoms that lived in harsh conditions under a seemingly permanent fog. Easily swayed by the miraculous technology that missionaries brought to their world, the planet's kings converted to the Imperial Creed and came to peace. A number of religious orders were established, and huge swathes of the population became monks, wholly devoting themselves to studying the Emperor and His saints. The priests and bishops came to rule the people, and the Carthaxian Diocese adopted the world as a place of faith. In the millennia to follow a great number of important religious texts were transcribed on Kayan, and millions of missionaries took to the stars to spread the Carthaxian Creed. Kayan was largely untouched by the problems that befell the Antonine Cluster during the 41st millennium as it had largely not moved on from its feudal technological base, and serfs still worked the fields by hand and the monks were not possessed of material trinkets and baubles. It was a serene and contented place, a situation that contributed to its quick capitulation in the face of an unexpected foe.
In 879.M41, the space hulk Imminent Retribution appeared in the void around Kayan, catching the Imperial Navy by surprise. What system ships could be pressed into service offered little resistance to the thousands of Ork craft that poured from its launch bays, and the system was quickly overwhelmed. The poorly armed militia that constituted the world's planetary defence force were butchered by the greenskins that fell to earth in roks and landaz. Two religious orders that had seen the rise and fall of Goge Vandire and the Nova Terra Interregnum come and go were wiped out in a ground war that lasted a mere four months. The Imperial response came too late to save Kayan, despite the destruction of the Ork fighta-bomma facility aboard the Imminent Retribution by Deathwatch Kill Team Carantus. The system remains quarantined by the Ordo Xenos, a few Throne Agents and Deathwatch Marines on covert missions observing and sabotaging the Ork war machine as it grows in strength. The Inquisition wishes to contain the Imminent Retribution to the system, but space hulks are known to follow no rules when it comes to transitioning to the warp, and many believe its departure is long overdue.