The Segmentum Obscurus, also known as the Segmentum Obscura, is the region of Imperial space to the galactic north of Terra. This Segmentum of the Imperium of Man is the home of the vast Warp Storm and Warp Rift called the Eye of Terror, the primary home for the Forces of Chaos in the Milky Way Galaxy. The Imperial Navy's Segmentum fleet headquarters is located in the Cypra Mundi System, which also serves as the central headquarters of the Imperial military's Segmentum Command. The Segmentum Obscurus is the area of the Imperium most often under deadly threat by attacks from the Eye of Terror and is also the site for all of Abaddon the Despoiler's Black Crusades. As such, it currently is home to the majority of the Imperium's armed forces, which were most recently mobilised to a state of readiness not seen since the Horus Heresy as a result of the 13th Black Crusade in 999.M41.
Agripinaa Sector - The Agripinaa Sector is located close to the Eye of Terror. It is a major producer of military materials for Cadia and is a linchpin in the overall defence of the Cadian Gate. In comparison to other Imperial sectors, it is a wealthy and prosperous area and produces many regiments for the Imperial Guard.
Cadian Sector - The Cadian Sector is the most heavily fortified sector in Imperial space, as it is the home of the Cadian Gate, the only clear route out of the Eye of Terror and into Imperial territory. The Fortress World of Cadia stands at the center of the sector and its people serve as the unflinching sentinels of the Cadian Gate. Portions of Cadia are currently occupied by the Forces of Chaos, following the indecisive outcome of the 13th Black Crusade in 999.M41. Cadia is currently circled by several Imperial Navy battlefleets and the maintenance of this space superiority is all that is keeping the Forces of Chaos from staging their first breakout into Imperial space in force since the end of the Horus Heresy.
Calixis Sector - The Calixis Sector is a sector of the Milky Way Galaxy that serves as the setting for the Warhammer 40,000: Dark Heresy role-playing game, which focuses on the actions of Imperial Inquisitors and their chosen Acolytes and Throne Agents. The Calixis Sector is an Imperial sector located in the Segmentum Obscurus on the northern edge of the known galaxy, near the Halo Stars and the Eye of Terror. To the trailing edge of the galaxy, the Calixis Sector is bordered by the hazardous territories of the Fydae Great Cloud and to spinward by the Scarus Sector. Bordering to coreward, its immediate neighbour is the Ixaniad Sector. To rimward lie the contested and unregulated Imperial frontiers of the Halo Stars where humans and xenos mix in the quest for power and profit in the infamous Koronus Expanse. Conquered by Lord Militant Angevin for the Imperium of Man almost two thousand years ago in the 39th Millennium during what became known as the Angevin Crusade, the Calixis Sector contains many heavily populated and important worlds, though it lies a great distance from the Imperium's core sectors. The sector's first Imperial Sector Governor was Drusus, one of Angevin’s most capable generals, a man now revered as a Saint of the Imperial Cult.
Scarus Sector - The Scarus Sector is a division of the Segmentum Obscurus close to the Eye of Terror. Its sector capital is the Hive World of Eustis Majorus. The Scarus Sector borders the Calixis Sector to spinward and its known sub-sectors include Helican, Antimar, Angelus and Ophidian. This sector is the theatre of operation for the renowned Inquisitors Gregor Eisenhorn and Ravenor. The Helican sub-sector is the site of the Helican Scism, a wave of rebellion, terrorism and sabotage that swept the area. The Ophidian Subsector is noted for being the site of the Ophidian Campaign (also known as the Purge Campaign), in which Battlefleet Scarus wrested control of the sub-sector from heretic forces. The campaign lasted from 240.M41 to 337/338.M41 — a considerable achievement in such a short timescale. Lately, the Scarus Sector has been plagued by invasions of Orks, who attacked en masse during the 13th Black Crusade (a campaign led from the Eye of Terror by Abaddon the Despoiler himself) and Greenskin infestation remains a very real problem in this corner of the Imperium of Man.
Gildar System - The Gildar System is a star system in the Segmentum Obscurus of the Milky Way Galaxy and it possesses an astonishing wealth of natural resources. The system consists of eight worlds, most of them uninhabited. The Silver SkullsSpace MarineChapter considers the system to be a part of their territory and no Imperial starship can travel through the area without their permission or escort. The world of Gildar Secundus was the site of a vicious battle between the Silver Skulls and the Red CorsairsRenegade Space Marines for control of the region known as the Gildar Rift. The Silver Skulls proved victorious and drove the Red Corsairs from the system, but at a very high cost. The Captain of the Silver Skulls' 4th Battle Company, Daerys Arrun, was slain in close combat by the Red Corsairs' infamous Chaos Lord, Huron Blackheart.
Cadia (Fortress World) - Cadia, officially known as Cadia Prime, is a terrestrial, Earth-like planet that has been classified as both a Civilised World and as the Imperium of Man's most important Fortress World by the Administratum. It guards the only known navigable route to and from the massive Warp rift known as the Eye of Terror, a passage called the Cadian Gate. The world's dangerous proximity to the Eye of Terror has made it necessary for the people of Cadia to heavily fortify the planet. Cadia is always the first target of the Chaos Warmaster Abaddon the Despoiler's assaults and Black Crusades, when the Forces of Chaos Undivided launch themselves from the Eye of Terror every few centuries in an attempt to break out and invade the Imperium proper as they did during the Horus Heresy. Cadia's natural environment is much like Terra's was millennia ago, with a large ocean covering 70 percent of the planet's surface. The land mass that does exist is divided between incredibly thick pine forests and vast glaciers. The planet is slightly cooler than most human-settled worlds but not to the point that it adversely affects growing conditions. Settled before the onset of the Age of Strife by a branch of humanity that eventually fell to the worship of the Chaos Gods and played a major role in the ultimate corruption of the Space Marine Legions, Cadia was re-settled sometime in the early 32nd Millennium by Loyalist humans of the Imperium. The world's landscape is dotted by strange black pylons of clear xenos origin called the Cadian Pylons. These devices were actually constructed by the Necrons millions of years ago to hold back the psychic energies of the Eye of Terror from the world, which was an ancient Necron military base during their war with the Old Ones. The Cadian Pylons are what create the unusual area of realspace stability known as the Cadian Gate near the Eye of Terror that is unaffected by the constant Warp Storms that surround that Warp rift. Cadia's location directly adjacent to the dangerous Eye of Terror has made it necessary for the people of Cadia to fortify the planet to an extent where almost the entire population lives in massive fortress-cities known locally as "Kasr". Thus Cadia has an odd mix of dense urban areas and vast open tundras and other natural landscapes. It is always against Cadia that Abaddon the Despoiler, the Warmaster of the Chaos Space Marines and the Forces of Chaos, fields his continual assaults or Black Crusades from his hold in the Eye of Terror.
Fenris (Death World) - Fenris is the Death World in the Segmentum Obscurus that is the homeworld and recruiting ground for the Space WolvesSpace MarinesChapter and of their PrimarchLeman Russ. Fenris takes its name from the mythical Norse wolf fathered by the god of tricksters Loki that would assault the Norse gods during the Final Battle of Raganarok. Most of the names of locations on Fenris are also drawn from the Norse mythology and languages of Old Earth. Fenris is situated in the galactic south of the Segmentum Obscurus, at the end of the Warp rift known as the Eye of Terror, from which come the Forces of Chaos to raid and pillage. Fenris thus lies at the forefront of the Imperium's defence against Chaos. The Space Wolves maintain the vigil that began many millennia ago at the close of the Horus Heresy and watch over a hundred other worlds besides. Their domain stretches far and wide across the scattered stars that girdle the Fenrisian star system, but it is the bitterly cold Death World at its heart that the Space Wolves proudly call home.
Medusa (Feral World) - Medusa is the Chapter homeworld of the Iron HandsChapter of Space Marines. The world of Medusa is a harsh realm of perpetual gloom, situated precariously close to the Eye of Terror in the Segmentum Obscurus. The sun almost never breaks through the dark and polluted sky, as it constantly churns over a land of frozen mountain ranges, interspersed with volcanoes and boiling hot geysers. The landscape is under constant flux, the shifting of tectonic plates forming new mountains and seas, and destroying them as quickly as they are created.
Mordian (Hive World) - Mordian is an Imperial Hive World in the Segmentum Obscurus that is the homeworld of the Imperial Guard's Mordian Iron Guard Regiments. Many of the Imperium's enemies have underestimated these ranks of colorfully-uniformed troops, only to find out that they are among the toughest and most uncompromising of the Emperor's soldiers. The people of Mordian are often of a dour and grim temperament, living as they do in perpetual darkness. The suicide rate on the planet is reportedly now the highest in the Imperium since the destruction of the Night LordsTraitor Legion's homeworld of Nostramo. Mordian also once came under a great assault by the Forces of Chaos that was defeated only through the extraordinary discipline and valour of the Iron Guard in one of the Imperium's finest moments. Mordian is tidally locked with its star, so that the Mordian day is the same as the Mordian year. As the planet does not rotate on its axis this means that like Luna, one-half of the planet is bathed in perpetual darkness and the other half burnt to a cinder by the glaring sun. This unusual planetological characteristic makes Mordian a truly miserable world to live upon. The planet's lack of rotation prevents any strong winds from developing due to the lack of a Coriolis Effect and as such the atmosphere is thick and dark. The world's billions of human inhabitants live in vast hive cities where all resources are controlled and distributed by the Mordian state. There is much discontent among the people, and the Imperial Guard Regiments raised from Mordian, the Mordian Iron Guard, spend much of their time simply keeping the populace in line when they are not deployed to Imperial war zones.
Scintilla (Hive World) - Scintilla is the capital world of the Calixis Sector of the Segmentum Obscurus, and is a thriving Imperial hub that supports the largest planetary population in the sector. It is dominated (some observers say “shared") by two vast hive cities, Hive Sibellus and Hive Tarsus, into which the vast majority of the planet’s groaning population are crammed. Despite the dominance of the two great hives, the “offspring” communities of Ambulon and Gunmetal City contribute significantly to the planet’s economic function. Scintilla is a world of splendours where the wealthy and powerful compete with ruthless appetite. Astonishing magnificence abounds, from the wondrous fashions of the hive nobility to the towering spectacle of the hives themselves. Landmarks like the Lucid Palace and the Cathedral of Illumination are famous throughout the sector.
Vostroya (Industrial Hive World) - Vostroya is an Imperial industrial Hive World in the Halo Zone of the Segmentum Obscurus, located beyond the Eye of Terror. The planet is considered an "Industrial World" by the Imperium of Man, because it does not quite meet the legally defined requirements to be considered an Adeptus Mechanicus-ruled Forge World, but it is not quite like an independently-governed Imperial Hive World either. Like all Forge Worlds, Vostroya has deep ties with the Adeptus Mechanicus, but because it is not ruled entirely by the Priesthood of Mars, it is not classified as one of their worlds. The origins of this unusual arrangement can be found in the Age of Strife, when Vostroya was the rare independent world that swore allegiance to Mars, rather than be settled by a colony of Tech-priests as was the norm for almost all of the other Forge Worlds. After declaring its allegiance to the Mechanicus, Vostroya quickly developed into a heavily industrialised world with the influx of Mechanicus personnel and technological knowledge. Under Imperial rule, the output of the many Vostroyan manufactorums now provide war materiel for the Emperor of Mankind's massive war machine.
Halo Stars - The Halo Stars are the ancient stars that encircle the outer edge of the Milky Way Galaxy, representing the last stellar clusters to be encountered before one enters the eternal, frigid night of the intergalactic void. The Halo Stars are believed to be the oldest stars in the galaxy and their circumference extends for approximately 200,000 light years around the galaxy, usually grouped in a cluster formation. The Halo Stars may also be intermixed into the shroud of dark matter that is known to extend beyond the visible disk of the galaxy for several hundred thousand light years. The Halo Stars are older than most can understand, and are characterised by mystery and danger. The Halo Stars have remained largely unexplored by the Imperium of Man and it is unknown what kind of habitable worlds, if any, may circle them. The Halo Stars have a very bad reputation amongst Imperial starfarers and it is believed by many, including quite a few Rogue Traders and Adeptus MechanicusExplorators, that those foolish enough to venture out into the Halo Stars never return. The Koronus Expanse is an area of the Halo Stars adjacent to the Calixis Sector in the Segmentum Obscurus that has begun to be explored by agents of the Imperium, led by the fleets of various Rogue Traders.