The Imperium Nihilus, also known as the Dark Imperium in Low Gothic, was the name given to the half of the Imperium of Man isolated from Terra after the formation of the Great Rift, the so-called Cicatrix Maledictum, during the climax of the 13th Black Crusade in 999.M41.
The Imperium Nihilus encompasses most of the Segmentum Obscurus and Segmentum Ultima, and was a defining feature of the astrography of the Milky Way Galaxy during the period known as the Noctis Aeterna of the 42nd Millennium.
The Dark Imperium was well-named, for beyond the Great Rift, the light of the Astronomican was obscured to the sight of Navigators and the Immaterium was roiled by the waxing might of the Ruinous Powers, making faster-than-light travel and astropathic communication extremely difficult, if not impossible.
Thousands of worlds trapped within the territory of the Dark Imperium, such as Baal, the Chapter homeworld of the Blood Angels Space Marines, came under renewed assault from the myriad enemies of the Emperor of Mankind, including the forces of Chaos and xenos like the Tyranids, Orks and Drukhari, the dark Aeldari kin.
Such was the turmoil during the creation of the Great Rift and the period of the Blackness following it that there could be no reliable accounts of all that transpired in the territory of the Imperium Nihilus. As limited communications returned, Historitors and Chronotechs struggled to understand incoming reports. With the influx of Warp energies into such vast regions of realspace, time passed strangely, speeding up in some sectors, slowing in others compared to the sidereal time experienced on Terra, at the heart of the Imperium.
These effects on the Warp from the birth of the Great Rift faded with the passage of time, allowing for the reestablishing of interstellar communications with some regions and somewhat more reliable Warp travel. This restoration of stability, combined with the onset of the Imperial counteroffensive known as the Indomitus Crusade led by the resurrected Lord Commander of the Imperium Roboute Guilliman through the first standard century of the 42nd Millennium, restored the grip of the Emperor over the Imperium once more.
However, the price paid proved steep. Slightly less than half of the one thousand Space Marine Chapters known to operate across the galaxy remained unaccounted for, and no less than twelve Space Marine Chapter homeworlds were reported as destroyed during the Noctis Aeterna and the equally bitter campaigns that followed.
Travel from the Imperium across the Great Rift into the Imperium Nihilus was extremely hazardous, and considered nigh-impossible, though the Indomitus Crusade fleet found a path through to come to the aid of the Blood Angels during the Devastation of Baal.
There was only one known passage -- the Nachmund Gauntlet. The Cicatrix Maledictum had at least one gap in realspace, although others were reported but not confirmed. The passage was near the Eye of Terror, which had been avoided by Navigators since the Great Crusade, for Warp Storms and strange anomalies had always made the region too dangerous to travel.
There, when the swirling nebula lulled, was revealed the Nachmund System, near to Agripinaa on the near side of the rift. Its Tyrant King, Kaligius, ruled over House Mandrakor, a household of Renegade Knights based on the Knight World of Dharrovar that had not been seen since the Horus Heresy.
Claiming Old Night had returned to plague the galaxy once more, Kaligius rejected communications from the Imperium, instead making pacts with Renegades and pirate fleets. Some Imperial starships have run the Nachmund Gauntlet successfully, but many more have been destroyed attempting the journey -- the ruined hulks of innumerable craft hang suspended in space as a vast voidship graveyard.
In the wake of the Devastation of Baal and the defeat of Hive Fleet Leviathan by the forces of the Blood Angels and the Indomitus Crusade, Guilliman declared that Lord Commander Dante, the Chapter Master of the Blood Angels, would serve as the Regent of the Imperium Nihilus while he concentrated on defending and strengthening the rest of the Imperium on the other side of the Great Rift.
Notable Sectors of the Imperium Nihilus
- Gothic Sector
- Askellon Sector
- Calixis Sector
- Finial Sector
- Ixaniad Sector
- Koronus Expanse
- Scarus Sector
- Screaming Vortex
Notable Worlds of the Imperium Nihilus
- Alaric Prime
- Belis Corona
- Cypra Mundi
- Kar Duniash
- Warhammer 40,000: Rulebook (8th Edition), pp. 50-53, 123, 134, 152, 164-166
- Dark Imperium (Novel) by Guy Haley
- Codex Adeptus Astartes - Blood Angels (8th Edition), pg. 23, "Hope from Horror"
- Warhammer Community - The Dark Imperium
- Warhammer 40,000: New Heroes for a Dark Imperium
- Imperium Nihilus - Vigilus Defiant (8th Edition), pg. 11