"The slave cares not whose hand wields the whip or why, knowing only the authority of its touch, the discipline of its voice and the certainty of pain."
- — A Cthonian Proverb
In the 30th Millennium, when a world was liberated -- or subjugated -- by the forces of the Great Crusade, or willingly joined the ever-expanding domains of the Imperium of Mankind, it was said that it achieved Imperial Compliance. The Dark Compliance is a cruel mockery of this noble endeavour. The term has since become a byword for countless atrocities, having been coined during the dark days of the Horus Heresy and the Warmaster Horus' rebellion against his rightful Emperor for those worlds that were subjugated militarily by the Traitors or somehow coerced to secede from the Imperium and join Horus' shadow empire.
Having conquered the entirety of sacred Terra during what was to be named the Unification Wars, the Emperor of Mankind looked to the stars, intent to reconquer those worlds humanity had lost during the baleful centuries of the Age of Strife. With the compact of Mars, the Emperor now commanded immense armies of human soldiery as well as the beginning of what would eventually grow into the Legiones Astartes, the Space Marine Legions. As the Emperor undertook His mission to free humanity from the Xenos and Warp-creatures that had nearly destroyed it, He conquered world after world, exterminating the mutants and aliens that had threatened Mankind's rightful place amongst the stars. Where human society had survived, it was made part of the Imperium, or rather it "Achieved Compliance with the Imperial Truth" as the parlance of those great days would have it. Sometimes this integration were achieved through diplomacy and the promise of a better future and the promised restoration of humanity's great stellar empire. Often it required a show of force to subjugate planetary rulers and dynasties that had been used to governing their worlds for centuries and even millennia. Even more often, Imperial Compliance had to be obtained by force of arms. In these great wars, tyrants were cast down and witches burned, planets purged of the foul taint of mutants, xenos and Warp-beasts such that eventually these campaigns of planetary conquest became known as Imperial Compliance campaigns or, more simply, rendering a world Compliant.
These Compliance campaigns were carried out by every military branch of the Imperium: from the countless regiments of the Imperial Army and the coldly-logical ranks of the Mechanicum Taghmatas to the towering representatives of the Titan Legions of the Collegia Titanica, but greatest amongst these were the Space Marines. Countless worlds were brought into the fold of the Imperium by these gene-enhanced warriors, at whose heads stood the eighteen Primarchs, the Emperor's own sons from whom the Space Marine Legions had been raised. Yet even these multitudes were not enough to police and control the entirety of the Emperor's domain, and while some of the Legions indeed erected mighty keeps and fortresses to keep watch over the newly Compliant populations -- most notoriously the Imperial Fists, the Iron Warriors and the Ultramarines -- many were the Primarchs who refused to squander their warriors in such "administrative" tasks. Yet peace had to be maintained, and to fuel the Great Crusade's ever-growing need of supplies, ships and manpower the Emperor devised a grand plan, the installment of the Great Tithe, which every world of the Imperium would have to submit itself to. In accord with the Tithe Grade established by the newly founded Office of the Administratum, every world would offer up its most precious resource for the Emperor: supplies, precious minerals, rare crystals or chemical compounds, weapons and ammunition, tanks and voidships or even just its manpower. Entire planetary populations were moved and sent to colonise planets far from their homeworlds, while the most troublesome and dangerous peoples were pressed into Imperial Army regiments and sent to the frontlines of the Great Crusade. For many more warlike cultures, such as the many Knight Worlds that populated the galaxy, this was in itself an immense recompense: the chance to gain glory and renown amongst the stars.
When Horus' true intent became apparent, the usurpation of the Throne of Terra, the Warmaster did not annihilate what had come before him, but would pervert what he had helped establish in new and terrifying ways.
"Fear not that I am come to demand your surrender. There is in me no such gift of mercy. I am come to deliver not words, but fire."
- — Baron Armelan, Emissary of the Warmaster Horus to the Planetary Council of Subinus
On many worlds the seeds of corruption had been sown long before the hand of betrayal struck at Istvaan. When the Warmaster Horus led his forces into civil war, hundreds of Imperial worlds not directly connected to his power base declared for his cause. On these planets and dominions, many scattered around the dangerous peripheries of the Imperium's frontiers, it was because his agents and those of his allies within the Mechanicum, the Alpha Legion and the Word Bearers, had already poisoned their hearts against the Emperor. Horus had been chosen to become the Imperial Warmaster by the Emperor precisely because of his diplomatic nature. It should then have been no surprise that he would have known how to use words and promises to sway the allegiance of entire worlds. Many hundreds more, from isolated and otherwise unimportant Imperial colonies to keystones of the Imperium's economy and military strength would fall under his sway as the war progressed in a process that soon became known as the "Dark Compliance."
To each world over which the Warmaster's shadow fell, a simple choice was given: total submission and surrender or total destruction and brutal subjugation -- slavery or death. There were to be no other options and the question would be asked but once. It was a perverse parody of the progress and glorious goals of the Great Crusade, but served as more than mere scorn for the Emperor's dream or even the vainglory of a tyrant, for there was an underlying method and intelligence beneath the apparently wanton savagery, far different from the pure and utter chaos and violence that define subjugation by Forces of Chaos in the late 41st Millennium. When one militant world or stubbornly Loyalist star system was punished by apocalyptic destruction for their brave defiance, such fear was created in others nearby that their surrender came as a rapid and forgone conclusion, often without a shot being fired in their defence. Even to those worlds that resisted, the Warmaster offered mercy and servitude once his victory was unquestionable. Each world that joined the Traitors' shadow empire added not simply territory but manpower, production capacities and supplies, feeding a war machine that was growing exponentially in power. Further, those worlds marked for death -- where possible -- can be seen to have been more strategically expendable compared to those spared to fall through fear, deception, corruption or treachery, or the measured if no less brutal decapitation strikes for which Horus' own Sons of Horus Legion had long been famed. It can also be observed that the devastation visited during the Dark Compliances was never so thorough that survivors were not left to spread the word of what terrors they had beheld, and of the price this new shadow emperor exacted for defiance of his will.
- The Horus Heresy - Book One: Betrayal by Alan Bligh, pg. 22
- The Horus Heresy - Book Four: Conquest by Alan Bligh, pg. 17