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

Silent, swift, surreptitious. Watchwords of the nineteenth legiones astartes; the Raven Guard. Reckoned masters of shadows, the Raven Guard (unlike most space marine chapters) specialize in stealth and guerilla tactics, placing efficiency and autonomy above all else in their actions.

One of the smallest space marine legions, the Raven Guard numbered only 80,000 at the height of the Great Crusade, a period where various other legions had swollen to much greater figures, such as the 150,000 of the Sons of Horus. Like many of the nascent legions, the Raven Guard served under the Emperor of Mankind during the Unification Wars on Terra, then later journeyed out into the galaxy under the Imperial banner. For much of their career they acted as akin to assassins, generally tasked with executing enemy VIPs and with the strategic destruction of high-value installations.  Like all the legions, they were reunited with their primarch, in this case, Corvus Corax.

The Raven Guard, along with their brother legions the Salamanders and the Iron Hands, were ambushed on Isstvan V during what became known as the Drop Site Massacre. While the Iron Hands' primarch Ferrus Manus had already rushed ahead to confront his traitorous brother Fulgrim, Raven Guard primarch Corax and Vulkan, the primarch of the Salamanders, were not immediately cut off by the traitor legions' surprise attack. Vulkan urged withdrawal to their drop ships and to dig in against the assault, while Corax counseled that they should retreat from the planet, as he perceived that the battle was hopelessly lost. A sudden artillery attack saw the two primarchs lose contact, with neither knowing if the other had survived.

Meanwhile, back on the Raven Guard's homeworld of Deliverance, garrison commander Branne Nev, alerted by apparent premonitions of Corax's death, decided to risk charges of insubordination by traveling to Isstvan V with a body of troops to reinforce his legion and primarch.

As the Drop Site Massacre dragged on, unbeknownst to Corax, traitorous Alpha Legion marines were able to disguise themselves as Raven Guard, infiltrating their ranks.

After months of intense fighting, the remaining Raven Guard were eventually cornered by a larger force of frenzied World Eaters. As Corax and his sons braced themselves for a final, impossible battle, air support rained down upon the traitors; Nev's relief force had arrived in the nick of time. The confusion caused by their miraculous appearance allowed the surviving Raven Guard the time and means to finally escape the planet. The legion had been devastated; of the estimated 80,000 legionnaires that hit the dirt of Istvaan V, less than 4,000 reached safety.

In the aftermath, Corax became obsessed with enacting plans of revenge against Horus and the traitor forces, but found himself in a position of extreme weakness. Desperate, he traveled to holy Terra to ask his father, The Emperor for help. The Emperor chose to bestow Corax with the scientific knowledge needed to rebuild his legion at a far faster rate than traditional methods would allow. However, the Alpha Legion infiltrators learned of this, and were ultimately able to sabotage the process, transforming the Raven Guard recruits into grotesque abominations.