Shadow Killers were combat veterans of the Raven Guard Legion who suffered from extreme stress and would become overcome by a suicidal fury and ignore any notion of self-preservation or observe standard combat tactics. Often, they would fight until death had taken them.
It was the Primarch Corvus Corax who organised those afflicted Legionaries into a specialised formation, known as the Shadow Killers, and employed them as shock assault troops and assassins. These Legionaries were the precursor to the Moritats who would later be introduced into the Legiones Astartes order of battle.
Amongst the Raven Guard and the XIXth Legion that preceded them, there was a tendency amongst combat veterans placed in situations of extreme stress to forsake all pretense of self-preservation, fighting with a terrifying silent determination and the sole aim of causing as many casualties and as much destruction as possible before succumbing to their foe.
This strange blood-soaked ennui that drove many amongst the Raven Guard to sacrifice themselves in battle was known among the Terran XIXth Legion as becoming "Ash Blind", while the Raven Guard native to the later Legion homeworld of Deliverance knew it as the "Sable Brand", a condition heralded by the darkening of the entire eye to black. Those warriors who survived their suicidal furies sometimes recovered, their brethren never mentioning the incident in a typically Raven Guard display of reticence, but more often were evermore plagued in combat by the urge to slay until death took them, paying no heed to tactical plans.
The XIXth Legion, in the days before their Primarch Corax's return, made little attempt to organise the Ash Blind on the battlefield, relying on the flexible nature of their tactical doctrines to make the maximum use of their suicidal urges when they took hold. The Raven Lord later turned this aberrant effect of his gene-seed into a valuable weapon in his tactical arsenal, forming those afflicted by the Sable Brand into units of "Shadow Killers" and employing them as shock assault troops and assassins.
It was the employment of such troops that allowed Corax to best his brother, Roboute Guilliman, in several simulated battles, an unexpected tactic that the lord of the Ultramarines Legion was quick to adopt into the Legiones Astartes order of battle, creating the Moritat as it was later known amongst the ancient Space Marine Legions.
- The Horus Heresy - Book Three: Extermination (Forge World Series) by Alan Bligh, pg. 153