When the Horus Heresy erupted, the Night Lords had no hesitation in joining Horus against the Emperor. The legion had already earned a reputation for ferocity and savagery and there is compelling evidence that the Emperor was already planning to disband the legion before the Horus Heresy made such action impossible. In any case, once they had sided with the Warmaster the Night Lords embarked on a campaign of terror and genocide that has rarely, if ever, been equalled. Even after Horus had been defeated the Night Lords continued to attack the Imperium from their homcworld deep in the wilderness area of space known as the Eastern Fringes, though increasingly without any discernible plan or motivation for their steadily more murderous attacks. Finally the Imperial Assassin M'Shen was able to infiltrate the Night Lords' base and slay their Primarch and with this act the Night Lords quickly stopped being an organised threat to the Imperium.

The survivors made their way to the Eye of Terror, where they continue to take part in raids on the Imperium to this day. All of the evidence appears to indicate that they do not worship any of the Chaos gods and have become instead cynical, hard-bitten and tnghtcningly ruthless warriors who fight for the pleasure of fighting and the material rewards it can bring. Some reports imply that this breeds a contemptuous attitude toward their more dedicated brethren, be they fanatical Chaos Space Marines such as Кhorne Berzerkers, or zealous loyal Space Marines like the Dark Angels or Ultramarines.

When Horus led his attack on the Emperor, the rebel Death Guard Legion was marooned in the warp where a mysterious contagion began to spread from ship to ship. The stinking pestilence bloated the gut, distended the flesh and turned its victims rotten from the inside. Eventually Mortarion became infected and in his delirium he called upon the powers of Chaos to aid the Space Marines. Mortarion`s fevered ravings were answered by Nurgle and Mortarion became Nurgle's Champion.

The Death Guard, newly dedicated to Nurgle, arrived in time to take part in Horus' attack on earth and fought in all of the major battles of the campaign. As disease began to disfigure the Death Guard, their appearance changed into the disgusting form they still bear today and the once tall and noble Space Marines of the Death Guard became Plague Marines of Nurgle. Following Horus' defeat. Mortarion led his Death Guard in a merry dance of destruction over a score of planets until finally retreating with the remnants of the legion into the Eye of Terror.

Following their retreat from the material universe, it becomes difficult to say with certainty what happened to the Death Guard. However, it seems likely that Mortarion received Nurgle's ultimate reward and became a fully-fledged Daemon Prince, ruling over a Plague world in the Eye of Terror. From this dark and slimy orb Mortarion launches fleets of Plague ships into the warp to carry their contagions through the galaxy.

The Emperor's Children were among the units assigned to crush Horus and his rebel Chapters on Istvaan V. During a parley the legion's Primarch. Fulgrim and his highest ranking officers were corrupted by the decadent pastimes that Horus and his Chaos worshippers offered. Drugged, pleasured beyond endurance and finally broken, they agreed to aid Horus. The rot quickly spread to the whole legion and the Emperor s Children willingly embraced Chaos in all its indolent depravity. The Imperial Cult was quickly supplanted by the more gratifying worship of Slaanesh.

As one of the Traitor Legions, the Emperor's Children invaded Earth, but took little part in the actual fighting around the Imperial Palace. When the assault failed, the Emperor's Children fled into the Eye of Terror with the rest of the Traitor Legions. They were the first to begin raiding Imperial worlds for captives and plunder. Their excesses soon knew no bounds and simple raiding could not supply enough raw human material for their orgies of worship. At this point it appears that the Emperors Children turned on the slaves and servants of the other Traitor Legions, an action which began a scries of bloody wars within the Eye of Terror. If there was indeed a civil war, it now seems to have been resolved or, at least, temporarily forgotten, for there have been proven cases of warriors of the Emperor s Children legion taking part in joint raids on the Imperium during the last several millennia.

The path of damnation for the Thousand Sons legion was longer than most. Even before the Heresy they studied arcane lore and sorcery despite the Emperor's warnings to stay clear of such matters. When Horus gathered his forces against the Imperium, the Thousand Sons at first tried to use their occult powers to warn the Emperor. Mistrustful of anything touched by Chaos, the Emperor declared them heretics and sent the Space Wolves to devastate the Thousand Sons' homeworld of Prospero.

Fleeing this onslaught the Thousand Sons sought the patronage of the Chaos power Tzecntch, greatest wielder of magic among the Chaos gods. Tzeentch favoured them well and the Thousand Sons escaped to the Eye of Terror, where they settled on the world which became known as the Planet of the Sorcerers. As ever with events that take place within the Eye of Terror, it is difficult to find confirmatory evidence, but it appears that as time passed many Thousand Sons declined into mutation and madness under the warping influence of the Lord of Change. The sorcerers became increasingly disturbed by the degeneration of their brethren until they eventually risked undertaking the drastic Rubric of Ahriman which purged the Thousand Sons of those members that were considered unworthy, and which created the strangely robotic Thousand Sons Chaos Space Marines that are encountered today.

The Iron Warriors once formed the Emperor's most able body of siege troops. The Iron Warriors' Primarch, Pcrturabo. excelled in siege and trench warfare above all else and his treatise on fortifications and their destruction formed the basis of several sections of the Tactica Imperium.

For reasons long forgotten, the Iron Warriors betrayed their oaths to the Emperor in the wake of the drop site massacres on Istvaan V and declared their allegiance to Horus. After Istvaan the Chapter split up to fight on a dozen other worlds and contingents of Iron Warriors led assaults against the Imperial Palace. Wherever Iron Warriors fought they built huge evil citadels in their wake and held them against all comers. Even after Horus' defeat the Iron Warriors were only removed from Imperial worlds at a terrible price. The surviving Iron Warriors were driven back into the Eye of Terror, but their skills arc still called upon for assaults on fortresses of the Impcrium today, as the destruction of the Cadians impregnable bastion line on Magnot Four-Zero well demonstrates.

The Black Legion is the only Traitor Legion to have changed its name in its ten thousand years of exile. The legion was originally created in the first founding as the Luna Wolves, but it was as the Sons of Horus that the legion fought in the Horus Heresy, serving as his praetorians throughout his campaigns. They were the first to remove the symbol of the Imperial eagle from their armour and banners and replace it with the Eye of Horus symbol which became such a dread icon of Chaos to the Impcrium. When Horus was defeated, factions in the other Traitor Legions later blamed the Sons of Horus for beginning the rout from Earth by retreating into warp space with the body of their beloved Warmaster, leaving the horde leaderless below.

Although it is very difficult to be sure, it appears that once in the Eye of Terror the Sons of Horus fought ferocious battles with the other legions for possession of key worlds and resources, fighting to re-establish themselves as the prime legion. Although evidence is fragmentary at best, it seems that these internecine conflicts eventually culminated in the destruction of the Sons' final fortress by a combined force of Traitor Legions. To add to the ignominity the Warmaster`s body was removed and cloned, much to the disgust of the remaining Sons. Denied their Warmaster, the Sons of Horus swore allegiance to Abaddon, captain of the First Company, as their new Warmaster.

Even before the Heresy the World Eaters were noted for their savagery and were censured by the Emperor for their use of psycho-surgery on new recruits to turn them into frothing madmen. Nonetheless the World Eaters were invaluable troops in the Great Crusade and fought at the forefront of all of the greatest campaigns. However, their ferocity made it a simple matter for Horus to quickly pervert the World Eaters' bloody legion rituals to the worship of Chaos.

Once renowned for their unswerving loyalty to the Emperor, the World Eaters became reknown for carnage and terror during the Heresy. They always fought in the vanguard of the Traitor Legions and their legion records show that it was they and not the Sons of Horus who first breached the walls of the Imperial Palace. The World Eaters retreated from Terra only grudgingly and fought their way to the Eye of Terror, carving a bloody swathe through anything that impeded them. All the evidence points to the World Eaters continuing and strengthening their blood traditions while in exile. All pretence of forming balanced, tactical forces has now fallen away and they are now armed entirely with pistols and close combat weapons, chain-axes and chamswords becoming the favoured tools of bloodshed.

The Alpha Legion was the twentieth and last legion created in the First Founding. Though the youngest, or perhaps because of it, the Alpha Legion sought to outshine its brethren in all things, as if to prove their worthiness amidst the older legions. Their pride led to a number of incidents where members of the Alpha Legion clashed with and even fought against members of the other legions in existence at that time. In particular there seems to have been a strong mutual hatred between the Alpha Legion and the Imperial Fists as they had clashed on more than one occasion prior to the Heresy.

When Horus made his pact with Chaos, the martial pride of the Alpha Legion was their downfall and they were among the first to join him. The legion clashed with loyal Space Marines on Istvaan V and in the campaigns thereafter, any need for restraint forgotten. Even after the Heresy failed the legion has continued to fight a covert war against the Space Marines and there, are dozens of well-documented instances of their raiding parties sallying out from secret bases to catch the defenders of Humanity unaware: sabotaging bases, attacking shipping fleets and destroying small outposts with deadly efficiency.