Chapter Text
Segmentum Solar
Great Crusade era
Smallest of the Segmenta, it comprised Terra and its hinterland; the Sol System sat at its heart and the other four were arrayed around it.
Legiones Astartes based in this Segmentum
Nominally, the Luna Wolves, from Horus's home world of Cthonia
Mars, headquarters of the Mechanicum, was Segmentum Fortress and ranked as equivalent in importance as the Legion homeworlds, or arguably even higher.
Reconquest era
From Terra toward the east. After the Custodianship, the western border of the Segmentum drew very close to Sector Solar, and the eastern border moved up against the impenetrable galactic core.
Segmentum Fortress: Official fortress is now the shipyards of Evraad, though unlike other Segmentum Fortresses its organizational significance is almost purely limited to military coordination. The Segmentum Fleet at Evraad, while large, is smaller than the Fleet of Mars which has primary responsibility for defending Sol. The Administratum Master of Segmentum Solar is based on Luna, and little other power rests on Evraad in any other respect.
Notable features
Prospero, home world of Magnus though now a Dead World, sits within Segmentum Solar
Cthonia, home world of Horus and Saint Uruk, still sits in Segmentum Solar. It is primarily significant as the birthplace of the Church of the Martyr and therefore one of the foremost worlds of the Imperial Faith. This strand, founded by Saint Uruk, worshiped both the Emperor and the fallen primarchs, Horus first among them. This was a minor church outside its regional strongholds around Cthonia, Prospero, Inwit, and Olympia, but was elevated in importance during Valdor's Founding of Faith.
Segmentum Obscurus
Great Crusade era
Segmentum Obscurus sat to the galactic north of Solar, from Chinchare in the west to Valhalla in the east.
Segmentum Fortress: Forge World Cypra Mundi
Legiones Astartes based in this Segmentum
Dark Angels from Lion El'Jonson's homeworld of Caliban, just south of the Halo Stars and the Rangda.
Space Wolves from Leman Russ's homeworld of Fenris, on the eastern fringe of the segmentum.
Reconquest era
After the segmental reorganization, the western border moved east to the Cypra Mundi system, and its de jure eastern border to slightly west of Baal. However, its eastern border with Segmentum Nihilus is the most heavily contested space in the galaxy, so the de facto border is constantly shifting. However, it rarely moves much further west than Valhalla or further east (and thus closer to the Eye of Terror) than Sable.
Segmentum Fortress: Forge World Lucius, perpetual strong point of the battlefront opposing the Eye of Terror
Notable features
The Storm of Wrath: A recurring severe Warp Storm in the neighborhood of the Askellon Sector. It lasts for a decade or more, then fades before reappearing anywhere from twenty to two hundred years later, always in the same general vicinity but rarely the same subsector; sometimes it has moved the breadth of two whole sectors.
Holscht, Fortress World, Shrine World, and the Segmentum headquarters of the Imperial Faith. It is dominated by the Cult Angelis Tenebrum and home to the closest thing that cult has to non-astartes leadership.
Caliban, left infested by Chaos after the actions of the Space Wolves, and left a shattered Dead World after the sanction of Exterminatus was levied on the remains, still sits in the Segmentum and is monitored by a task force of the Last and First for the potential that it could be used for sorcerous ritual to attack the legion.
Segmentum Pacificus
Great Crusade era
Second-smallest and least-populous segmentum, it started at the western edge of Segmentum Solar between Hydraphur in the north and Gehenna in the south, and stretched west out to the edges of the Astronomican's light.
Segmentum Fortress: Forge World Hydraphur
Legiones Astartes based in this Segmentum
Initially, the Night Lords, from Night Haunter's home world of Nostramo, in the far southwest of the segmentum. They later became fleet-based, as directed by the primarch at the same time as he resumed use of the name Konrad Curze.
The Hydra had an unusually strong presence in this Segmentum.
Reconquest era
The eastern border moved inward and now encompasses the Forge Worlds of Voss, Phaeton, and Graia. The northern border passes slightly west of Forge World Cypra Mundi, then east of Hellion and Opuscalus Noctis.
Segmentum Fortress: Still Forge World Hydraphur (now more centrally located)
Notable features
Nuceria, home world of the primarch who was known as The Fettered, sits in the far northwest of the segmentum.
Seraphina, Cemetery World and Segmentum headquarters of the Imperial Faith
Segmentum Tempestus
Great Crusade era
It sat south of Segmentum Solar, starting from the Taras Division and stretching past Tallarn to Antagonis and the west edge of the Hungering Gyre. In the west, its border passed near Krieg and Chiros.
Segmentum Fortress: Bakka
Legiones Astartes based in this Segmentum
Raven Guard from Corvus Corax's home world of Deliverance
Before Rogal Dorn was named Praetorian of Terra, the Imperial Fists were based on Inwit
The Death Guard, from Mortarion's home world of Barbarus
After Perturabo was named Vigilus Marchensis, the Iron Sages, from the Iron Line
Reconquest era
The western border now passes due south near Graia, with Solstice one of the westernmost systems. To the east, the nominal border passes southwest from Olympia, east of Chogoris and the former location of Nocturne, leaving the galaxy just east of the Hungering Gyre. The practical border is the Iron Line, which has its northern terminus at the Maelstrom and, like the western border, heads nearly straight south, passing between Chemos and Chogoris and very near the former location of Nocturne before ending at the west side of the Gyre.
Segmentum Fortress: Still Bakka (now more centrally located)
Notable features
Chemos, former home of Fulgrim and his Emperor's Children, now a Dead World, sits in the north of Tempestus
Ophelia, Shrine World and at times galactic headquarters of the Faith, lies in the Phobian system in Segmentum Tempestus. It is named for Saint Ophelia, founder of the Mysteries of the Undying Emperor, a church which drew heavily from voidgoers and the ruling class in the days before the Founding of Faith incorporated it into the Imperial Faith. It is second only to Holy Terra in importance to the modern Faith and has hosted a number of Faith institutions since the Thorian Reforms.
Lycaeus, briefly known as Deliverance, and its primary Kiavahr, home to the Ravenspire and the Homewing order of the Raven Guard
Ultima Segmentum
Great Crusade era
The Ultima Segmentum began at the eastern edge of Segmentum Solar, on an arc passing near Elysia, Prospero, and Nikea, before extending outwards and east past the galactic core to Ultramar in the southeast and the Ghoul Stars in the northeast. It was abolished during the Custodianship, with the western half being divided into its neighboring segmenta and the eastern half redesignated Segmentum Nihilus, i.e. outside Imperial control.
Segmentum Fortress: Kar Duniash
Legiones Astartes based in this Segmentum
The Reforged, from Ferrus Manus's home world of Medusa
The Blood Angels, from Sanguinius's home world of Baal
The Chained Hounds, from their muster world of Bodt, just coreward of Chondax
The Iron Sages, from Perturabo's home world of Olympia, until he was given the duty of Vigilus Marchensis.
The Thousand Sons, from Magnus's home world of Prospero
The Emperor's Children, from Fulgrim's home world of Chemos
The White Scars, from Jaghatai Khan's home world of Chogoris, a.k.a. Mundus Planus
The Word Bearers, from Lorgar's home world of Colchis
The Salamanders, from Vulkan's home world of Nocturne
The Ultramarines, from Roboute Guilliman's home polity of Ultramar
Segmentum Nihilus
Reconquest era
Segmentum Nihilus is practically defined as the wide band to the galactic east of the core, from Bale, Neverlight, and the Evermaw north through the Thirteen Realms and the Dominion of Storms to Honourum, Angelis, and the Eye of Terror. The technical definition is defined in relation to a certain band of angles measured from the galactic core, but it is effectively the same. It is the area which it is infeasible to protect from the depredations of the forces of Chaos, where any attempt to hold ground mandates a two-front war and so it can only be contested back and forth.
Segmentum Fortress: The Phalanx, headquarters of the Obsidian Fists, who have fought a perpetual crusade in Segmentum Nihilus since their founding.
Notable features
The Eye of Terror, to the galactic northeast of Baal at a radius from the core about two and a half times that of Baal. The largest Warp Rift in the galaxy and second-largest Warp Storm, it predates the Great Crusade and now is home to three full traitor legions and portions of three more.
The Evermaw, created as the lasting effect of the Ruinstorm ritual which engulfed Ultramar in the largest Warp Storm ever seen. It shrank somewhat in the decades after the Heresy War and created a true Warp Rift at its heart, to which the Ultramarines and Word Bearers retreated after their defeat; this smaller but more intense version was named the Evermaw by the astartes of the Iron Line, upon realizing it was not going to dissipate further. Since a century after its shrinking began, however, both storm and rift have been growing, slowly but steadily; they passed the peak extent of the Ruinstorm in M35 and seem to be adding a roughly constant amount of volume per century.
The Signus Accord: A bastion of hope between hells, the Signus Accord is a cross-species alliance, principally between the T'au and the exiled Forge World of Triplex Phall. Far beyond the ability of the Imperium to protect them or even support them, Triplex Phall, already self-sufficient by necessity, sought to build a more durable, secure region of space which could withstand raids without intervention by the Obsidian Fists or other crusading astartes orders. The Warp-resistant T'au, a young but rapidly-developing race, made contact with them in late M33, having achieved spaceflight in the preceding two centuries and slowly expanding outward from their home world near Konig Secundus. They sought to convert the Mechanicum magoi to their philosophy of the Greater Good, but their devotion to the Omnissiah, though very much heterodox by this time, remained far too strong. They did, however, share knowledge, with both sides receiving technical knowledge and Triplex freely sharing what they knew of Chaos, the dangers of the galaxy, and particularly the heretic astartes raiders. Sobered T'au sought and found corroboration of their claims, and their Ethereals made the decision to make solid alliances even with those who declined to join the Greater Good, a principle they named "Nont'shas Ko'vash", literally "Shadow-Time's Worthy Cause" but usually translated into Low Gothic as "the Desperate Times Principle". They, their client races, and Triplex Phall signed an accord (brokered by the living-stone humanoids called variously the Durazi and the Demiurg) to come to each other's aid, share technology as much as was feasible, and pool information and agents in the investigation and expunging of Chaos taint from their worlds. This accord with xenos is frowned on by many of the loyal astartes who operate in Segmentum Nihilus, but they recognize that Triplex Phall would have fallen long ago without it, so other than assisting with the smuggling of Masked Hands and Imperial Faith priests onto the human-peopled worlds of the Signus Accord, they have left the T'au and their small empire alone.