The Coronid Deeps, formally referred to as Coronid Thule, is a region of Imperial space that encompasses the frontier border areas of the Segmentum Obscurus and Ultima Segmentum, and constituted some of the most industrialised regions of the northern Imperium in the early 31st Millennium.
This highly important manufacturing sector is still considered to be both vital and strategically valuable to the Imperium of Man. Between 006-008.M31, this region was the site of much conflict between Traitor and Loyalist forces during the Horus Heresy. Much of the Coronid Deeps region would be incorporated into the later Imperial Gothic Sector.
The Coronid Deeps' roots lie deep within the Dark Age of Technology, when Mankind first took to the stars and began its sojourn into the wider galaxy beyond the Sol System. Many worlds in the various regions of the Coronid Deeps were settled at this time, as the human settlers transformed their worlds into habitable and industrious centres of human civilisation. Then this region of the galaxy was cut off from the rest of humanity by massive Warp Storms which raged for thousands of Terran years. Each world was forced to fend for itself to survive the horrors of Old Night a tumultuous period of human history also known as the Age of Strife.
Before the coming of the Great Crusade, many regions within the Coronid Deeps suffered under the inhuman overlordship of the Mitu Conglomerate. The Conglomerate, whose core worlds would eventually be purged by the fires of Exterminatus, was a cabal of several psychically-empowered pseudo-actinaric xenos species believed to have evolved from a common stock. Warp-capable and with advanced biologic and arcane technologies, the Mitu Conglomerate had forged a pocket interstellar empire, forbidding any attempts at space flight among their subject worlds and ruthlessly purging any psykers found within their domains. They periodically demanded a culling of the population and a tribute of flesh to provide them, it was believed, with the basic components to construct their technologies.
Contact with the broader reach of Mankind was reestablished with many of these human remnant civilisations when Imperial Expeditionary Fleets of the Great Crusade first began to explore the Coronid Deeps in the early 9th century of the 30th Millennium, at a time when much of the Coronid Deeps were still entirely uncharted and under the Conglomerate's control. In answer to the existence of such xenos creatures there could be no response but total annihilation, and the war the Imperium fought to end the Mitu Conglomerate was protracted and bloody.
The conflict itself was fought largely in the void as a series of brutal close-quarters boarding engagements, interspersed with sudden and world-shattering planetary assaults by both sides, which involved the forces of several different Expeditionary Fleets, and would require no fewer than three Space Marine Legions and hundreds of Solar Auxilia cohorts to successfully prosecute.
The liberation of many of these star systems was a turning point for humanity, and for the masses who had lived so long under the oppression of inhuman intelligences and the constant threat of death, Imperial Compliance was accepted with open arms. The various regions were soon put under the rulership of an Imperial Commander or the custodianship of the Mechanicum. The Coronid Deeps became a highly productive and vital strategic region of the new born Imperium during the latter years of the Great Crusade, becoming a shining beacon of Imperial civilisation and all the Great Crusade had set out to achieve.
This region was effectively cut away from regular direct contact with the Segmentum Solar owing to the rising tide of Warp turbulence which would culminate in the so-called "Ruinstorm", which followed on from the dark events at Calth in 007.M31. What came to be known as the Manachean War was a campaign carried out by those under the direct command of the Warmaster Horus during the latter part of 007.M31.
It was both an early example of the Warmaster turning his forces to seize entire sectors of the Imperium he deemed valuable to his cause, and a demonstration of the speed and brutality with which such campaigns were accomplished. It can be seen in retrospect to have provided a template for scores of such campaigns which came after it, both in terms of strategy and the savage consequences involved for those worlds which fell into the Traitors' grasp throughout the time of the Horus Heresy's Age of Darkness. The Conquest of Manachea, led personally by Horus and his inner cadre of commanders, was itself the centrepiece of an interlinked series of campaigns, fleet actions and local wars which swept like storms through the celestial region of the Coronid Deeps.
Manachea itself was the jewel of the region; a highly-populated, highly-advanced Hive World of the first rank which sat at the centre of its own well-developed sub-domain of attendant star systems called the Manachean Commonwealth. As such, it offered the Warmaster much that would be of value in the ongoing galactic civil war, not least of all strategic control of the region, and vast human resources to exploit as well as industrial capacity that could be readily turned towards the war effort.
Nearby sectors also included the mineral-rich Cyclops Cluster -- home to two fully-fledged Mechanicum Forge Worlds, the vital Armada Imperialis fleet anchorage of Port Maw, several independent Knight Worlds, and a score of Agri-worlds and other colonies that could be used to feed and supply Horus' armies on the long march to Terra. These prizes were not, however, unguarded. Although the Traitors' agents had already been at work in the region, the bulk of its worlds and military strength was already set against him, the region's Imperial Commanders alerted by the wildfire warning that had spread after the news of the Istvaan III Atrocity had reached Terra.
That such a warning had been issued in the region (whereas many other sectors and domains had been kept ignorant at first of the betrayal) was a result of the frontier's own proximity, in relative terms, to the Istvaan System itself; Port Maw being used as a layover and resupply base for a number of the fleets despatched so disastrously to punish the Traitors at Istvaan V.
With only a few notable exceptions, the garrisoning of the Imperial worlds of the Coronid Deeps was provided for by the Imperialis Armada and regiments of Solar Auxilia Pattern Excertus Imperial Army formations. The Solar Auxilia Pattern was particularly suited to the defence of the region as it was equipped and formed to fight in hostile environments and participate in void battles, both of which were common in this far-flung domain of the Imperium, while the high technological base of worlds such as Manachea and Agathon made the requirements of Solar Auxilia Pattern regiments possible to maintain in a manner that was simply untenable on the majority of Imperial colony worlds.
These defenders had shown themselves repeatedly able to withstand the assaults of xenos raiders, rebel uprisings and, on occasion, wholesale invasion -- at least until reinforcements arrived. They were well provided with arms, and warships, and numbered in the hundreds of thousands in terms of active troops. They had never though been measured against the unthinkable opponent, the Imperium's own Space Marine Legions.
So it was that in the closing days of 006.M31, terrifying reports, quickly suppressed, reached the governing courts and councils of the Coronid Deeps that the unthinkable had occurred; that the Traitors had crushed those sent to bring them to heel at Istvaan V in the so-called "Drop Site Massacre." These reports were at first scoffed at and wilfully disbelieved, until the first few battered ships, Loyalist survivors of the void battle at Istvaan, began to limp in, seeking sanctuary from the hounds of Horus at their heels.
Fear came then to Manachea, to Agathon, to Mezoa and a hundred other worlds greater and lesser along the frontier. Requests for aid and instruction from Terra were lost on blinding squalls of Warp static, and with the Warp Storms behind them and the black void of the frontier before them, the worlds of the Coronid Deeps could do little but wait for the devil to come for them from the dark.
By the dawn of 008.M31, the Coronid Deeps, once a stable outpost of the Imperium, had become in part a fractured series of Traitor-ruled domains in a wider galaxy riven with strife. But this too was soon to be challenged as the spirit of war that had been unleashed would not be so easily assuaged, and for some victory was to prove fleeting. The counter-invasion of Numinal and the catastrophic destruction visited on Gethsamaine would only be the start of a new phase in the ongoing war for the Coronid Deeps. What would follow throughout the Coronid Deeps would become a protracted and bloody series of campaigns and battles with no certain victor.
Outposts were raided, planets laid to waste, convoys ambushed and murderous reprisals taken; the tables frequently turning so that the hunters became the hunted and back again, and where one side gained an advantage, it was quickly squandered or laid low by sudden reversal of fortune. Loyalist and Traitor clashed again and again; forces came and went from the region as the tides of the unquiet Warp allowed and battle waxed high and waned in bloody exhaustion, only to be taken up again.
So it was with the Coronid Deeps; so it was to be all across what was once the unified Imperium of Man. A grim age of war had been born at Istvaan III that would never end in the lifetimes of those who were caught in its bloody embrace, an age of war that would outlast the Horus Heresy, outlast the Great Scouring and beyond, an Age of Darkness.
- Coronid Prohibition Zone - An astra-navigational hazard of considerable size and uncertain borders, the Coronid Prohibition Zone, also commonly known as the Coronis Abomination and popularly to members of the Navis Nobilite as the "White Darkness." It borders the central northern edge of the Coronid Deeps region and, by popular wisdom among the void-faring, presents the most obvious reason for the dividing point of the ancient Segmentae in human stellar cartography. This apparently all-but-empty region of space holds up neither spectacle nor warning to either the naked eye or Auspex, while in the Warp it apparently represents itself as "white out" to the Navigator's vision, akin to the blindness a man might encounter if trapped in a blizzard -- a literal "nothingness". The cause or consequence of the danger this region entails remains entirely unknown. By direct order of the Emperor, the Coronid Prohibition Zone was established, explicitly forbidding deliberate entry to this mysterious and deadly region of space, and a number of autonomous armed watch stations were created, roughly proscribing the region's borders. These stations establish and police the cordon around the zone, and dispatch Warp-capable hunter-killer Gholem-ships to engage and destroy any vessel or object detected leaving the zone.
- Coronid Reach - The furthest anchor point of Imperial civilisation in the region, the Coronid Reach is rightly regarded as the very edge of the dark frontier of galactic space, beyond which little may be said for certain. Its centre of power is the Hive World of Agathon, which dominates the region via its economic and military might, and represents the final conquest of the 60th Expeditionary Fleet during the Great Crusade, before its disbandment. This highly autonomous Imperial region comprises a cluster of sixteen inhabited star systems.
- Cyclops Cluster - The Cyclops Cluster is one of the most heavily industrialised sectors in the northern Imperium, featuring as it does within a single stellar volume two Forge Worlds as well as sixteen primary grade industrialised worlds and ninety-three distinct resource extraction zones. The Cyclop Cluster's economic output and strategic position is important to the larger Imperium. Though rich in minerals and ores, it is deficit in complex organics, with relatively few life-sustaining worlds among its stars.
- Cyclothrathe Holdfast - The former sovereign domain of the ancient Mechanicum, the Cyclothrathine Holdfast was founded on two of the few life-sustaining planets left in a desert of Dead Worlds wrought by the hand of the Great Crusade. Set up in the ashes of what were once the core systems of the xenos empire of the Mitu Conglomerate, the Holdfast was created both to oversee scavenging operations and to serve as a weapons-testing facility, as well as to ensure that all trace of the alien life forms which had made up the Conglomerate were utterly purged from existence. This region was ruled over by the Cyclothrathine Mechanicum, which had always been an insular and aloof brand of the Mechanicum, even when it was still counted amongst those loyal to the Imperium, few were those who readily trusted them. They had garnered a reputation for ruthlessness and aggression, both in defence of their domains from any xenos that crossed the frontier, and in pressing any claims they made to resource rights and territory. During the Horus Heresy, the Warmaster Horus played on the jealousy and ambition of the Cyclothrathine Mechanicum to whom Mars was at best an unwanted and distant authority. Their name would become a byword for infamy and atrocity amongst the entire Imperium, as its position within the Dark Mechanicum and the reputation of its dire warlord, Archmagos Draykavac, were both cemented in the blood of countless planets.
- Grail Abyss - A celestial wasteland ranging between the galactic spiral arms, sparsely littered with stable star systems, rogue planets and the bled-out remnants of cosmic detritus and other shifting navigational hazards, the Grail Abyss is aptly named. Forming the extreme border of the Segmentum Obscurus and the edge of known space, beyond its borders lie only the drifting stars and systems accessible purely owing to the vagaries of relatively stable Warp passages from the galactic core (including such star systems as the now infamous Istvaan, and Ultima Ectosa, the furthest star across the northern galactic plane known to hold an Imperial colony). Owing to the shifting topography of the Grail Abyss, and the psychic emptiness in local Warpspace that gives it its name and makes predictable interstellar travel in the region unreliable, travel within the Grail Abyss is indeed more difficult in the main than reaching those furthest stars of Mankind, and likely was so long before the Imperium's birth. The human worlds of the Abyss are therefore few and far between, no more than candle-points flickering in a sea of night.
- Manachean Commonwealth - This aggregate of advanced and populated worlds, set at the junction point where the Segmentum Obscurus ends and the northeastern sweep of the Ultima Segmentum presses onwards to its outermost limits was, at the dawning of the war of the Horus Heresy, without doubt the most strategically vital area of the Coronid Deeps. Clustered with inhabitable stars, many of which had possessed human populations since before the Age of Strife, the Commonwealth served as a lynchpin of commerce and trade in this region of the burgeoning Imperium. Vitally, it was also a powerhouse of industry and military endeavour which held the shape of the burgeoning Imperium's borders during the Great Crusade. The two principal axes of this power in the region were the Manachea System itself, comprising several inhabited worlds with a population reaching into the many billions, and Port Maw, a colossal naval base still under construction when the Warmaster's treachery came to pass, and set upon an ancient artificial planetoid believed to be of xenos construction and vastly ancient, long predating even the rise of the Aeldari Empire in the galaxy.
- The Horus Heresy - Book Four: Conquest (Forge World Series) by Alan Bligh, pp. 10-11, 18-19, 37, 40, 43, 53-55, 57, 62-63, 65-76, 115, 117, 124-125, 140, 158