"In the darkness all men are equal, save those who embrace it."

Silent, swift, surreptitious, the Raven Guard are the nineteenth legion of space marines from the First Founding. Unlike most space marine chapters; they specialize in stealth and guerilla tactics. The Raven Guard place efficiency and autonomy above all else. They do this out of survival just as much as preference, as they are one of the smallest First Founding chapters, numbering only 80,000 during the height of the Great Crusade as opposed to other legions such as the Sons of Horus who numbered around 150,000. Like many first founding chapters, The Raven Guard served alongside the Emperor of Mankind during the Unifications Wars on Terra until being united with their Primarch Corvus Corax. Acting as the Emperor's assassins, they are generally tasked with executing enemy VIPs and strategic destruction of high-value installations.

Loyalists, they were one of chapters ambushed on Isstvan III, alongside the Salamanders and Iron Hands during what is known as the Drop Site Massacre. While the Iron Hands' primarch Ferrus Manus had already rushed ahead to make the arch-traitor Horus answer for his crimes, The Salamander's and their primarch Vulkan where traveling alongside the Raven Guard and Corax when the traitor legions launched their surprise attack. Vulkan wanted to withdraw to their drop ships and dig in, while Corax felt they needed to retreat from the planet as the battle was hopelessly lost. This disagreement and a sudden artillery attack saw the two primarches separated and unaware if the other brother lived. Meanwhile, back on the Raven Guard's homeworld, Garrison commander Branne kept suffering from premonitions of Corax's death. Unable to make contact, Branne risked insubordination by traveling to Isstvan three to check on his legion and primarch.

During the fighting and unbeknownst to Corax, the traitorous Alpha legion were disguising themselves as fallen Raven Guard and infiltrating their ranks.

After months of intense fighting, the remaining Raven Guard were eventually backed into a corner, sandwiched between a cliff drop and an infinitely larger force of frenzied World Eaters. As Corax and his sons braced themselves for a final but impossible battle, air support rained down on the traitors. The ensuing confusion coupled with the off-world support allowed the Raven Guard the time and means to finally escape the planet. Heavily devastated however, as the legion numbered less than 4,000. A loss of over 75,000 legionnaires.

Obsessed with thoughts of revenge against Horus, Corax and his legion traveled to holy Terra to ask his father, The Emperor for help. The Emperor bestowed Corax with the scientific knowledge needed to rebuild his legion at a far faster rate than using traditional methods. The Alpha Legion learned of this however and had their implanted spies sabotage the process, transforming the Raven Guard recruits into grotesque abominations.