After the Heresy, in the light of the Warmaster's treachery, vast structural changes were made to the fighting forces of the Imperium. Titan Legions and Imperial Guard regiments were forbidden their own transport vessels so that in the event of rebellion they could not leave their own star system without wholesale treachery on the part of the navy as well. In the so-called Second Founding, the mighty Space Marine Legions which had been tens of thousands strong in the Great Crusade were divided up into "Chapters", each no more than a thousand strong. Never again would one man wield as much power as Horus and the other Primarchs of the Space Marine Legions.

The events of the Horus Heresy have long since passed into myth within the Imperium. Ten thousand years of history have obscured those dark days when Horus almost enslaved mankind with the shackles of Chaos. Indeed, records of the full horror of the Heresy are only preserved now by the daemon-hunting Inquisitors of the Ordo Malleus, and perhaps within the memories of the Emperor himself. The involvement of Chaos in the Heresy has been carefully concealed by the High Lords of the Imperium, fearful that others might tread the same path of damnation as the renegade Warmaster.

But within the Eye of Terror time flows differently and the same traitors who howled their praises to the gods of darkness before the Imperial palace still live to this day. Their defeat gnaws at them like a cancer and their hatred of the Imperium which they helped to forge burns undimmed. These deadly warriors who were tall, powerful Space Marines, proud warriors clad in plasteel and ceramite armour, have become cynical and embittered reavers determined to destroy what they once protected.

Within the Eye, the Traitor Legions fight constant wars amongst themselves for gene-seed, slaves, resources or martial honour. New Chaos Space Marines are recruited from the most dangerous. heretics that are drawn into the Eye by the lure of Chaos, or else selected from the masses of slave-warriors who fight eternal battles for the amusement of the Dark Gods. The implantation of recruits is a brutal affair, quite unlike the carefully measured program of development used by Imperial Space Marines. Whether the candidate lives or dies is left to the will of the Chaos Gods. Initiation rites are similarly debased and savage, ensuring that only the toughest of the initiates ever survive.

Millennia of jealousies and infighting have broken down the Legions into companies and warbands of varying size, each led by their own Captain or Champion of Chaos who pursues his own destiny. The most fervent individual worshippers of the Dark Powers have banded together to form squads blessed by their patron gods: Plague Marines of Nurgle, Berzerkers of Khome, Slaaneshi Noise Marines and Tzeentchian Sorcerers. Others have given themselves over to possession by daemons, allowing the monstrous spawn of the warp to venture out into reality clad in the flesh of a powerful host.

In the ever-changing delirium of the daemon worlds most Chaos Space Marines have survived the worst of the warping influence of Chaos by keeping their sense of purpose. They have not forgotten, nor forgiven, the mortal universe which exiled them and still exact a bloody vengeance when the tides of the warp carry them back there. They are carried forth by ancient warships that have survived from the time of the Heresy, but are now encrusted with millennia of baroque decoration and scarred by old battle wounds. The Legions also capture drifting space hulks and refit them to use as vast battle barges carrying thousands of troops.

The ships of the Traitor Legions are driven by the fickle tides of the warp and can appear anywhere in the galaxy at any time. The Chaos Space Marines descend upon an unsuspecting Imperial planet, plunder it and raze anything they cannot take with them before retreating into the Eye of Terror. Though local defence forces may struggle against the invaders they are doomed unless Space Marine reinforcements are on hand. Then the Chaos Space Marines have a joyous opportunity to visit woe and destruction on their weakling younger brothers, perhaps the true objective of these random raids.

Major invasions are instigated in one of two ways. Chaos Cultists may plunge a world into anarchy and revolt, generating a ripple of worship in the warp which attracts the attentions of the Chaos Gods. By casting auguries and making divinations Chaos Sorcerers can guide an invasion fleet to the Cultists' world. Once there the Chaos Space Marines will fight alongside the Cultists to capture the whole planet in the name of Chaos. Only then will the Cultists realise that they have summoned up entities who will enslave them as surely as the iron rule of the Imperium.

But even bloody conquests like these are little more than pinpricks against the galaxy-wide Imperium of Mankind, the blows which shake its very foundations are dreaded "Black Crusades".

Perhaps once or twice in a millennia a truly great Champion of Chaos will arise in the Eye Terror. Through the power of his implacable will and the favour of the Dark Gods this Champion can weld together an unsteady alliance between the infernal regions of the Eye. How the Champion brings the crusade together depends his nature and his patron god. Some use manipulation, others extortion, others domination, others intimidation. Most simply use all of the considerable powers at their disposal.

Preparations for a Crusade can take years, or weeks depending on the whims of the gods. The forges of the hell worlds belch out armour and weapons for the chosen one's followers, daemon engines are aroused from their dark slumber with blood sacrifices, factions vie for command of the massed ranks of crusaders or are mercilessly crushed into obedience.

When the Black Crusade is launched the Eye Terror vomits forth the diabolic hordes of Chaos: armies of daemons, rank upon rank of huge, twisted monsters; numberless masses of Cultists; wild tribes of Beastmen; ancient and terrifying Chaos Titans. Spearheading it all are the Chaos Space Marine Legions, united in their lust for booty and their desire to bring destruction upon the hated Imperium.

The Imperium keeps strong forces stationed around the Eye to fend off these invasions. Entire Titan Legions, Space Marine Chapters and massed regiments of Imperial Guard defend the most vital systems in close proximity to the Eye. But even powerful fighting formations like these cannot guarantee victory over the infernal throng. All too often the black tide of Chaos expands and recedes leaving entire systems ravaged and burned. Whole planetary populations are irrevocably tainted by Chaos, cities and industries are crushed by the thunderous pounding of diabolic engines of destruction, uncounted citizens are dragged away to serve as slaves an playthings to the damned souls and their daemonic masters at the edge of reality.

Every city ruined, every planet burned brings the Imperium a little closer to dissolution. In an Imperium of a million worlds how much can a single world matter? Enough to have to defend each one against the infernal host, enough to bring the curse of Exterminatus upon those that bend the knee and bow down to daemon-kind. A Black Crusade may come crashing forth from the Eye only once in a thousand years, but the damage it inflicts can never be undone.