So I thought that I'd do a bit more expansion on the origins of the StormCrows. Since they have their Origins in the Great Crusade I figured I'd dive into that a bit since I'm gearing up to start doing their Horus Heresy progenitors. (Some will be made up since I can't find any lore, feel free to correct anything wrong).
Originally the Raven Guard drew the bulk of their recruits from the Asiatic Dustfields, but not all. The 12th Chapter of the Raven Guard were drawn from the Scandanavian Reaches (Modern day Iceland, Faroe Islands, Norway, Sweden, Finland, Denmark Netherlands, The Baltic States, and Northern Russia west of the Urals), ancestral grounds of the feared Viking Rievers of the early Age of Terra, and continued to be a breeding ground for hardened peoples ever since through into the beginning of the Age of Strife.
The 12th Chapter, The StormCrows (so named for their resemblence to the frequent and furious tempests which wracked their Terran homelands), was among the number of the XIXth legion that pacified the Asiatic Dustfields during the unification wars, and continued to serve faithfully. They were a harsh group, never willing to shy from a fight or a challenge. They were consummate warriors, always seeking to prove themselves. Moreover they were group minded, and knew that to survive was to trust their brethren, and so the 12th Chapter gained the reputation for being fearsome and unbreakable. Every fallen brother would increase their wrath, every attack would strengthen their defense. They excelled as shock troops under Warmaster Horus, specializing in raiding tactics and finding weaknesses.
Once Corax was found, and began purging the legion of Terran-born astartes, the 12th Chapter had to prove itself. They were too heavy handed, against the defeated, for Corax's liking, their brutal efficiency lacked Corax's vision of justice. Corax would not exile them immediately, as the 12th Chapter would show that it wished to conform with Corax. So Corax would give a cadre of his close companions, Deliverance-born astartes who had helped liberate the moon, to aid them in embracing the vision of Corax. The 12th would be exiled, with the caveat that they would be welcomed back to the fold if they showed that they had fully embraced Corax's justice. It was this shame that caused them to color their Insignia red, marking themselves as dead to the legion, until they could return.
The campaign they embarked upon during the great crusade to prove themselves worthy of the raven lord, would see them change drastically. It was not their methods in war which gave Corax pause, it was how they viewed the defeated. The 12th Chapter, Would soften their harsh view, beginning to treat the liberated not with suspicion, but welcome them. One of the regions they liberated would go on to become their recruiting grounds, the Skaði Cluster. It was also this campaign which would see them gain the name Öskumenn or "Ash men", and earn them their enduring reputation. The tyrants which ruled the cluster did so with a bloody hand. Slavery and mass executions were common, The people lived in fear and then came the armor clad ghosts. They tore through the defenses, burning their way through the capital, killing all who would terrorize the people. The Öskumenn gave arms to the liberated, and encouraged them to fight for their freedom.
The Deliverence born astartes would deem the StormCrows ready to return, but the StormCrows felt differently. They felt that merely liberating these worlds was not enough. They needed to see to it that these people would remain free, and began to train their militia. Some of this militia would follow the StormCrows to the stars and fight beside them in the latter days of the great crusade and through the Horus Heresy. Legends would arise, as the StormCrows had made their home in the cluster, about the ghostly, ashen-armored, men who fought beneath the raven banner sweeping through and punishing the wicked overlords. These legends would also tell that those who dared to look upon the cavalcade would be swept up to join them.
The 12th Chapter would rejoin their legion during the Horus Heresy, Serving Corax as shock troops and raiders. Their Red Badge and Ash-Stained armor serving to distinguish them from their sable armored kin. They would split from the legion once more during the second founding, going off to parts unknown to hunt the traitors and the infuence of chaos wherever it might be found.
They would not resurface until mid M35, When the Ordo Malleus encountered them exterminating a Chaos cult and the Daemons it had summoned. The StormCrows had been on the world already for some time, waging a shadow war for the world's very soul. Suddenly millennia old rumors and sightings of grey glad space marines bearing the symbol of a blood red raven made sense. After the world was cleansed, the Inquisition questioned the Astartes. They were asked their founding and name. The StormCrows had knowledge of the various foundings, and they answered that they were a second founding chapter, the StormCrows. Only they would know their true nature as the 12th Chapter of the vaunted XIXth Legion...
To this day, they see themselves as sons of Corax, before any other division. It is a secret which they only share with the current chapter master of the Raven Guard Chapter of the Adeptus Astartes, they are Væringjar ("Sworn Men") sworn to be Corax's talons, and to serve his sons, as they had sworn to defend the Imperium to Lord Commander Guilliman himself, upon The ArchTraitor Horus's vile betrayal...