The Mitu Conglomerate was one of the countless xenos empires that rose to power and preyed upon human worlds during the Age of Strife. Powerful and possessing devastating weapons and technologies, the Mitu Conglomerate comprised scores of worlds and held dominion over several vassal star systems, both human and alien. Their empire would eventually be crushed during the Great Crusade after a protracted and bloody war that would be led under the flag of the Primarch of the Imperial Fists, Rogal Dorn, and the contribution of the 7th Expeditionary Fleet of the I Legion, the Dark Angels.
The records of the Great Crusade contain precious little information on the Mitu and their empire -- the alien horror being too complete to warrant anything less than a writ of extermination by the Imperium. However, much was learned through their interactions with several human cultures in the Segmentum Obscurus, most notoriously their actions within the domains of the Manachean Commonwealth, which had suffered for many Terran centuries under the baleful tutelage of the xenos, and the Forge World of Mezoa and its allies of the Imperial Knight Household of House Hermetika. At the pinnacle of their might, the Mitu Conglomerate posed a serious threat to the Imperium of Man's expansion, effectively dominating a great deal of the Coronid Reach. It was held in check only by the Orks of the Grail Abyss and the might of Forge World Mezoa in the Cyclops Cluster.
Almost nothing is known of the Mitu Conglomerate's inception or early history. What few archives or fragments remained from the Dark Age of Technology does not mention them, which leads to the conclusion that their empire emerged sometime after Old Night had taken its hold on humanity. Before the coming of the Great Crusade, the Manachea System held a technologically advanced society, a survivor state of the ancient human diaspora which, while it numbered in the hundreds of millions, also 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. Typically, they resembled fungoid sea anemones with legs. Though the amount and orientation of limbs varied, they were bisymmetrical, aside from their misshapen, tentacled heads. Warp-capable and with advanced biological and arcane technologies, the Mitu Conglomerate had forged a pocket empire, forbidding any attempts at space flight by their conquered peoples and ruthlessly purging any human psyker within their domains.
From Manachea, as well as other subjected star systems (both human and alien), the Mitu 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. In answer to the existence of such 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 the Manachea System was a turning point in this great war, and for the masses who had lived so long under the oppression of inhuman intelligences and the constant threat of death, Imperial Compliance was taken up with open arms. Instituted as a commonwealth of the three inhabited worlds of the Manachea System under the rulership of a single Imperial Commander, Manachea prospered and soon came to dominate a score of nearby star systems as a regional capital, becoming a shining beacon of Imperial civilisation and all the Great Crusade had set out to achieve.
The Mitu were not a unified people or even a single species, but an alliance of different xenos civilisations that are believed to have evolved from a common stock. As such, all Mitu were pseudo-actinaric entities evolutionarily analogous to the sea anemones of ancient Terra that proved psychically-empowered, highly intelligent and masters of both biological and arcane technologies. Descriptions of their physique have sadly all been lost, though they are confirmed to have resembled walking, fungal sea anemones, with some variation between different breeds.
Located within the former Cyclotrathine Holdfast, over centuries the Mitu Conglomerate expanded deep into the Coronid Reach and the Cyclops Cluster. Imperial scholars know this from the archives of Mezoa and even distant Damaetus III/II, the home of House Vyronii, which both had to repeatedly fend off attacks from this growing alien empire. Other human worlds like those of the Manachea System were, however, less fortunate and were overrun by the xenos. Where it encountered other populations, both human and alien, the Mitu did not push for expansion but culled the existing populations into submission, exacting a periodical tribute of flesh under the menace of more thorough purging. Those worlds that lived beneath the thumb of the Mitu Conglomerate did not suffer as harshly as under the iron-shod rule of the Orks or the sadistic depredations of the Dark Eldar, but they were slaves nonetheless. However the Mitu did not interfere much with the planets' governance: apart from culling the nascent human psyker population and forbidding any attempt at space flight, the planets were mainly left to govern themselves. The ruthlessness the Mitu displayed in their identification of psykers indicate that at the very least they had a notion of the perils that lie within the Warp, although it is unclear if they knew anything about the existence of the Chaos Gods. Perhaps they simply sought to deny a powerful weapon to the populations they oppressed, or safeguard their technology.
On a technological level, the Mitu were a formidably advanced empire, mastering both space flight and interstellar travel. The damage sustained by the mighty Felweather Keep on Damaetus III/II constitutes a testimony of their military might, the imposing fortress proving no match for the high-power directed energy weapons used by the Mitu. It would seem that although technologically advanced, the technology of the Conglomerate was mostly organic and activated by psychic impulse, thus explaining both the Mitu's reoccurring need for tithes of lives and flesh and the cull of human psykers. Such horrors could naturally not be permitted to live by the Imperium.
When the Imperium ventured forth in the Coronid Deeps it encountered scores of worlds subjugated by the Mitu. Mezoa was by then involved in its own modest xenocidal campaign, which had already lasted for many decades, and on Damaetus III/II House Vyronii readied itself to make a glorious final stand before being overwhelmed by a new tide of alien horrors. Determined to carry out the total annihilation of the Conglomerate, the Imperium waged a bloody and protracted war against the Mitu. Several times the use of Exterminatus-class weaponry was sanctioned, for Mankind could ill-afford to let such monstrosities live. The conflict itself was fought largely in the void as a series of brutal close-quarters boarding engagements interspersed with sudden and world-shattering assaults by both sides which would mobilise the forces of several Expeditionary Fleets and require no fewer than three Space Marines Legions and hundreds of Solar Auxilia cohorts to prosecute. The liberation of the Manachea System would prove to be the major turning point of the conflict and heralded the complete extermination of the Mitu, whose core-worlds were all subjected to Exterminatus. To commemorate this victory, the Imperial Fists Legion raised one of their fortresses, the great Bastion Kvet on the world of Manachea Lux, where the decisive battle against the Mitu was won, swearing that the worlds would never again fall under xenos dominion.
In the final days of this conflict, the Mechanicum war-barque Cyclopean Mind was so badly damaged that it was left behind on the savage world of Cyclothrathe to become the birthstone of the Forge World which would gain so much infamy during the Horus Heresy.
The Imperium has thus far been unable to identify the original homeworld of the Mitu and it is likely that it never will as no resources can be spared to research the matter. Logic dictates that it would have to be one of the worlds ravaged by the Great Crusade that now form the Cyclothrathine Holdfast. However, the two principal worlds of this domain, Cyclothrathe itself and the Mining World of Arrian, seem not to have been colonised by the Mitu due to their inimical nature to life.
- The Horus Heresy - Book Four: Conquest by Alan Bligh, pp. 66, 72, 76, 124