Cypra Mundi is an Imperial Forge World and the primary Imperial naval and military base, or Segmentum Fortress, for the Segmentum Obscurus, the Segmentum Majoris of the Imperium of Man that includes the Eye of Terror. The Segmentum Fortress is directly overseen by an official of the Administratum with the grade of Master. Located on the northern fringe of the galaxy, Cypra Mundi is also the traditional home for Imperial Navy officers in the Segmentum, who are usually chosen from amongst the aristocracy of Cypra Mundi.
When Abaddon the Despoiler launched his 13th Black Crusade in 999.M41, a massive fleet was despatched from Cypra Mundi to reinforce the Imperial Navy, which had been fighting an ongoing battle against the seemingly endless waves of Chaos vessels. The fleet from Cypra Mundi staged at Belis Corona in preparation for a massive Imperial counteroffensive into the Cadian Gate, just outside the Eye of Terror.
Following the destruction of the Fortress World of Cadia by the Forces of Chaos during the 13th Black Crusade, the Eye of Terror was no longer restrained by the pylons that had dotted the world's surface. The result was the birth of the Great Rift, a massive Warp rift that spread across the galaxy from the Eye of Terror to the Hadex Anomaly in the Eastern Fringe, essentially rending the territory of the Imperium of Man in half. To those on the Terran side of the rift, it is a tainted scar stretching across the sky. To those unfortunates on the far side, such as Cypra Mundi, in the region now named the Imperium Nihilus or the "Dark Imperium," it is something much worse -- the very gates of Hell.
Little is known of the early history of Cypra Mundi, save that it was more than likely discovered and colonised by Mechanicum Explorators during the Dark Age of Technology. Early in the Great Crusade, the armoured formations of the XIII Legion (before it was known as the Ultramarines) halted an Ork advance on the ash plains of Cypra Mundi, and in doing so saved a city of millions from a savage death, successfully bringing another world into the fold of the burgeoning Imperium.
The economies of entire sub-sectors are devoted to supporting the manufacture and supply of the Imperial Navy's warships, principally in regions which serve the greater Forge Worlds such as Mars or Cypra Mundi. Within its vast orbital shipyards, Cypra Mundi has become notable for manufacturing the Overlord-class Battlecruiser, an oft-difficult vessel to construct, but faithful and fierce in its service to Mankind. The design is an ancient but successful one, and new examples of the class are still commissioned every solar decade or so. Though extremely rare, Cypra Mundi is also known to occasionally produce capital-class warships for extremely wealthy individuals or dynasties with the financial wherewithal to commission the manufacture of a bespoke Cruiser.
- The Curse Entropic (865.M41) - A Chaos fleet of Iron Warriors and Daemon Engines unleashed a destructive machine-curse upon Cypra Mundi. Soon the Forge World runs with rivers of molten metal, its surface dotted with a million burnt-out machines. Just as all seems lost, the Cult Mechanicus coordinated a psalmic banishment that lifted the curse, leaving the Chaos Space Marines bereft of their daemon allies. The planet is cleansed within a Terran week.
- Codex: Adeptus Mechanicus (8th Edition), "Bastions of the Machine God," pg. 12
- Codex: Cult Mechanicus (7th Edition), "The Quest for Knowledge," pg. 38-39
- Forces of the Imperial Inquisition: Collector's Guide, pg. 4
- Imperial Armour Volume One - Imperial Guard & Imperial Navy, pp. 217, 230, 236
- Rogue Trader: Core Rulebook (RPG), pg. 315
- Rogue Trader: Battlefleet Koronus (RPG), pp. 22, 24, 67
- The Horus Heresy - Book Five: Tempest (Forge World Series) by Alan Bligh, pg. 73
- White Dwarf 139 (UK), "Space Fleet: Interstellar Travel - The Segmentae Majoris," by Jervis Johnson & Andy Jones, pg. 17
- White Dwarf 140 (UK), "Space Fleet: The Stellar Fleets," by Rick Priestley & Simon Forrest, pg. 50
- Warhammer Community