Sarum was a former Mechanicum Forge World that was home to an isolated Mechanicum station during the Great Crusade of the late 30th Millennium. It was located in the Golgotha Sector of the Ultima Segmentum perilously close to the permanent Warp Storm known as the Maelstrom. Sarum was a barren planet that lacked an atmosphere but was still home to extensive manufacturing facilities controlled by the Mechanicum. During the Age of Strife, the world was settled by the Mechanicum and transformed into an outpost of the Priesthood of Mars, but the station's Tech-priests soon found themselves isolated and embattled by the inhabitants of the Maelstrom. On its own against raiders, xenos, and Abhumans and without aid from Mars, the Tech-priests of Sarum became isolationists and founded a deviant cult of the Machine God under the leadership of the Magos Redjak. Redjak solidified his so-called Crimson Priesthood's power over Sarum and used the technological prowess of the Mechanicum that he commanded to gain political control over much of the Golgothan Sector by the end of the 30th Millennium. However, when Sarum faced a massive invasion by the Abhuman Brotherhood of Ruin and their Ork allies, the Tech-priests of that world pleaded for aid from the Imperium of Man's Great Crusade and finally received it in the form of the 13th Expeditionary Fleet of the World Eaters Legion, led personally by the Primarch Angron. In the ensuing Imperial campaign that became known as the Golgothan Slaughter, the Brotherhood of Ruin and their Ork mercenaries were annihilated by the World Eaters. However, Angron next turned his wrath on the Crimson Priests of Sarum, scouring the planet and claiming it as his forward operating base for the next eleven years in a campaign against the Crimson Priests' remaining empire in the sector.
The first known target to suffer the wrath of the World Eaters Legion after their Primarch Angron took command of the XIIth Legion, sometime in the last two centuries of the 30th Millennium, was the stellar wasteland of the Golgothan Sector. The Golgothan Sector was sited near the galactic core in the Segmentum Ultima close to the realspace Warp rift known simply as the Maelstrom. The breaking of the Siege of Sarum would soon become a cornerstone of the World Eaters' legend and serve as a portent of the dark path yet to come. Driving his Navigators perilously close to destruction in the Warp, Angron ordered his warships to form a single strike force aimed at the Sarum System. They came upon the besieged Adeptus Mechanicus station world of Sarum on the edge of the Maelstrom, founded during the Age of Strife and long cut off from aid. Sarum was being besieged by the Abhuman empire known as the Brotherhood of Ruin who wished to take the Mechanicum's technological secrets for themselves.
The Tech-priests of the Mechanicus' Redjak Cult had endured alone for ages, subjected to the privations of raiders and enemies all around them, with only intermittent contact with distant Mars and the Forge World Anvilus, requesting aid that had never yet managed to reach them. The World Eaters arrived as their saviours, smashing into the heart of the besiegers. Though dramatically outnumbered and outgunned, the World Eaters drew into close formation and smashed through the enemy armada. Once the enemy line of defence was breached, the World Eaters' fleet unleashed swarms of gunships and Drop Pods upon the planet's surface. In the meantime, baying for blood, Angron and his World Eaters ravaged the enemy armada, plunging in close so that the wild fire of their foes struck their own ships as often as those of the Imperials. Boarding torpedoes and assault rams screamed out from the World Eaters' vessels and slammed into the hulls of terror ships and Abhuman vessels, disgorging the World Eaters in their unstoppable, inhuman rage, turning the enemy ships into charnel houses.
Meanwhile, on the surface below, Abhuman cyborg troops and their Ork mercenary lackeys died by the thousands, unable to coordinate a defence against this unforeseen direction of attack. The battle raged on for hours, and the World Eaters took horrific casualties but fought on nevertheless. The Abhuman warlords sensed at last that the battle was turning in their favour and called upon reinforcements from across the planetoid's surface to aid them. It was then that a false dawn flared blood-red in the skies above. Seconds later, a Cyclonic Torpedo barrage smashed into the surface all around the World Eaters, who fought on. In their wake came hundreds of gunships and assault rams that represented the World Eaters' second wave, led by Angron himself. These warriors descended upon the Brotherhood of Ruin like a god of wrath. Behind them came 17 great black metallic cylinders which disgorged the towering Titans of the Legio Audax, which strode forth, weapons blazing.
Seeing the shadow of the Omnissiah's wrath descending, the Redjak Mechanicum emerged from their fortifications and speared the fleeing enemy with barbed harpoons and dragged them back for their masters to rend. The Siege of Sarum had been broken and the Redjak Mechanicum swore to Angron and his Legion directly many oaths of fealty and entered pacts of mutual protection and support. This provided the World Eaters and their Techmarines with a ready source of resupply and armament far outside the Imperium's inner sphere. The campaign that was to follow would last eleven standard years and see no fewer than 48 worlds and outposts ravaged and destroyed by the World Eaters, and 7 separate dangerous xenos species rendered extinct as the XIIth Legion's fleet cut a swathe of destruction through the wastes of Golgotha, and even ventured into the perilous fringes of the Maelstrom itself. Of course, this campaign also destroyed the miniature stellar empire that the Crimson Priesthood of Sarum had established in the sector, but they were no longer in any position to gainsay the Imperium that had saved them from destruction.
Several of the accounts of the Great Crusade that the Inquisition has unique access to make mention of the machine domain of Sarum, though they provide scant details save that the airless planetoid was home to an aberrant order of machine adepts and was the site of an overwhelming assault by the World Eaters Legion of the Legiones Astartes. Having collated numerous texts on the subject of this world, Inquisitorial savants have uncovered a link between it and a number of void clanner legends long told throughout the Golgothan Wastes. Though each story differs in many regards, they all appear to describe a wandering planetoid which is home to a crimson-clad order of hell-smiths whose faces are dominated by lamprey maws lined with razor-sharp steel teeth. It is not known if the long lost machine realm of Sarum was subjected to some unknown Warp phenomenon that has caused it to manifest at different times and places across the Golgothan Wastes and potentially beyond. While outlandish, such events are far from unknonw to the Inquisition. Furthermore, in assaying these tales that the Warp Ghosts are able to predict or even cause manifestations of this ethereal Hell-Forge, acting as some form of dread ferrymen as they steer the long damned foundries across the wastes and bring death and destruction upon the heads of those who dwell near the void along the coreward borders of the Segmentum Solar.
- Imperial Armour Volume Thirteen: War Machines of the Lost and the Damned, pg. 13
- The Horus Heresy - Book One: Betrayal, by Alan Bligh, pp. 99-101