Dorn's Darkness is a genetic deficiency suffered by the Astartes of the Excoriators Space Marine Chapter, a Successor Chapter of the Imperial Fists. The Excoriators believe themselves to be pure of heart but not of blood. As former members of the Imperial Fists Legion and now as a Chapter of their own, they are not alone in their experience of genetic deficiency. The Space Wolves and the Blood Angels, as well as the brethren of other Foundings, carry the flaws of their genetic heritage on to new generations. The Excoriators are afflicted by a genetic curse known as Dorn's Darkness. When Dorn's Darkness takes one of their number it might appear to the untrained eye as merely a wretched palsy: a slackness of the jaw, a tremor of the limb, a blankness of the eye. But those who survive it report the experience as a living nightmare, a sleeping wakefulness in which they relive the bottomless woe of Dorn’s most trying time—the grievous mortal wounding of the Emperor of Mankind during the final hours of the Battle of Terra. This is both the enetic blessing of the Sons of Dorn's Primarch and a curse upon his sons. It is to know the possibility—for even a second—of an Imperium without the Emperor. To feel what Dorn felt. The profound misery of a Primarch. The paralysing fear that even a man as great as Dorn experienced, for himself and for Mankind, over the Emperor’s broken body after the end of his battle with Horus. Whilst enthralled by the Darkness, its victims cannot speak or communicate. They cannot feed themselves or take water and seem feverishly insensible to everything happening about them.
Those Excoriators who fall victims to the Darkness were left in the care of the Chapter's Santiarch, or Senior Chaplain. The Santiarch Balshazar offered the victims of the Darkness a spiritual treatment that either cured them or ultimately resulted in their deaths. He had the afflicted interred within a decorative stasis casket that was transported from the Chapter’s Reclusiam on their homeworld of Eschara. The casket was beaten from dull Adamantium and the box had the dimensions of a sarcophagus and the extravagant garniture to match. Its frontispiece featured a raised depiction of the Emperor. Even though the casket stood upright, it represented the Emperor as prone, maimed and broken, as he was immediately following his confrontation with Horus at the end of the Battle of Terra. Balshazar’s solution to the affliction of the Darkness was to create a spiritual darkness of his own within the stasis sarcophagus. It was the most solitary of confinements, where no self-respecting Excoriator need look upon his own weakness and invalidity and where he might summon the strength of will to banish the Darkness of the Primarch and recover his sanity. It is not known whether or not the Excoriators are the only Imperial Fists Successor Chapter afflicted by this genetic deficiency, or if other Sons of Dorn suffer in silence as well.
- Legion of the Damned (Novel) by Rob Sanders