Space Marines - The Grey Legions: The first among the Space Marines were hand-picked men drawn from the Emperor's personal bodyguard. These volunteers were subjected to surgical, genetic and psychological modification.With rigorous training and appropriate mental conditioning they became not only immensely strong and tough, but iron-willed and disciplined, no longer prone to the psychological damage that normally affected Humans facing the stress of constant combat.They were an unstoppable force whose loyalty to the Emperor was unflinching. Quickly the process was refined and systematised, and the numbers of the new enhanced warriors, at first armed and armoured as the Thunder Warriors had been, grew swiftly.The first Astartes were organised into twenty distinct regiments numbering no more than a few hundred warriors each. Although it remained a dire secret at the time, it is now widely believed that this division was more than a merely administrative one, as each regiment contained variant "gene-seed" encoding drawn from a different primogenitor primarch.This often manifested its influence in subtle and unexpected ways, not least of all in influencing the psychological character of the genetically enhanced warriors.With the regiments expanding rapidly into full Legions with the intake of new blood from the areas of Terra that had already joined cause with the Emperor, the new warriors quickly eclipsed and replaced the mighty but far less-disciplined and mentally unstable Thunder Warriors. As the proto-Space Marine Legions were unleashed in the latter days of the Unification Wars, victory followed victory in quick succession.As time went on, the Space Marine regiments became Legions as the Emperor recruited young men from amongst the newly conquered tribes of Old Earth and the hundreds of Astartes in service to the Imperium swiftly became tens of thousands.These superhuman troops dominated the final days of the Wars of Unification, easily defeating all their Terran opponents and forcing those among the Tech-priests of Mars who had intervened in the conflict on Terra to delay the Emperor's victory to sue for peace.But in truth, the victories of the early Space Marines created a new problem within the Imperial fold. The Emperor had been right to be worried about His earlier creations, the Thunder Warriors.One ancient source claims that even before the Unification Wars had ended, the Thunder Warriors, already jealous of their replacements' long lives, at last realised that their creator had cursed them with short lifespans as a result of their imperfect genetic augmentations. In their rage and fear, they turned upon Him for what they saw as His betrayal.It was a cadre of several hundred Custodians of the Legio Custodes, the Emperor's bodyguard, even then believed to have been led by the legendary Constantin Valdor, and accompanied by several thousand prototype Astartes of the I Legion of the newborn Space Marines, that stood in the Emperor's defence, carrying out a merciless culling of the obsolete and rebellious gene-soldiers.Despite such tragedies, the Space Marines fought with righteous zeal and it was they who first referred to their mission as a "Crusade." By their efforts, for the first time in unrecorded millennia, the Earth was united under the rule of one man.The armour they wore was not new, but the same partially powered armour that had evolved on Old Earth and was worn by the elite of both the Emperor's armies and the techno-barbarian tribes that had fought against Him.Some of this "Thunder Armour," first named for the Thunder Regiments that were the Legions' forebears, was newly forged, but the Emperor's warsmiths also took or cannibalised many suits from the armouries and corpses of conquered foes.As if to mark a break from the wars of the past, the armour of the first Astartes was cast in storm cloud grey, and bore only the thunderbolt and lightning marks of Imperial Unity.Over time, the Space Marine Legions gained their own marks of distinction and character. Names, Emperor-given in some cases, others by the primarchs, came to replace the Legions' original numbers, with many Space Marine companies seeking to single themselves out from their brother Legions.Battle honours were accumulated and the effect of each Legion's character worked upon them, so that as the Legions expanded to conquer the galaxy, storm cloud grey became granite, silver, viridian, sable, gold, ocean, ash or ice, and by the time of the Triumph of Ullanor, the "Grey Legions" of the Unification era were gone, lost to history.