Dark Angels - Fall of the Thunder Warriors: As with all the Legiones Astartes, the Ist Legion was intended as a replacement for the brigades of the Thunder Warriors, that unstable experiment in the creation of proto-Astartes which by the middle years of the Unification Wars had run its course. The Thunder Warrior regiments had been the tool needed for their time, an unrefined and savage weapon to match the grim tyrants and debauched potentates that had inherited Old Earth during the Age of Strife. Yet, in comparison to the Legiones Astartes they were a rough breed, powerful to be sure, perhaps even individually more so than their new transhuman kin, but unable to quell their fury to work in unison. They were a mob, a storm of fury and blades that rolled over its foes, while the Legiones Astartes were a true army and in their unity could withstand any onslaught.Tales and theories beyond count surround the Thunder Warriors and their sudden disappearance from the Imperium's history, most being fabrications and sophistry of the worst kind, though a few warrant further regard. One such tale is of note, a tale told among the nomadic tribes that inhabit the wastelands around Mount Ararat and speaks not only of Thunder Warriors but also of an army "clad all in gray, dark as the clouds of a storm and branded all as one."These warriors, so say the hill nomads, awaited the return of the few surviving Thunder Warriors, exhausted by their victory, and rather than giving salute to their valour cut them down in a thunderous volley of bolt and plasma. This tale is far different from the heroic tale told by Remembrancers to mark the end of the Unification Wars, but in keeping with the brutal necessities of the Emperor's far-reaching plans. It is possible that the warriors "branded all as one" could be the Ist Legion, who all bore the sigil of their numeration bold upon their battle-plate, for battles of that Legion were oft-removed from Imperial record, the details expunged and forgotten.Actual proof, however, does not exist, not even in the archives of the Imperial Palace. Yet, several anecdotes from the later years of the Great Crusade do seem to support the theory, all related to a near-obsessive preoccupation with the hunting down and destruction of those few renegade Thunder Warriors that survived and fled Terra, over and above the standing orders of the Imperial Court to suppress all such renegades.On at least three occasions, fleets of the Ist Legion altered course in order to engage and destroy Thunder Warrior survivors, often making wide detours based on simple rumours of such activity. Such an obsession is notable for what was an otherwise stoic Space Marine Legion and would seem to hint at a more personal interest in the resolution of any encounter with surviving Thunder Warriors, perhaps even a sense of shame in having allowed any to escape some previous engagement.Another ancient source claims that even before the Unification Wars had ended, the Thunder Warriors, already jealous of their replacements' long lives and near-immortality compared to their own inevitable descent into madness and death, 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 the Emperor for what they saw as His betrayal.It was a cadre of several hundred Custodians of the Legio Custodes, the Emperor's lifeguard, even then believed to have been led by the legendary Constantin Valdor, and accompanied by several thousand prototype Astartes of the Ist 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.However, given the renowned secrecy of the Ist Legion and the frequency with which their campaigns and deeds were obfuscated by Imperial decree, it is unlikely that the truth of this matter will ever be known.