The Age of Darkness is the name later chroniclers gave to the years of the Horus Heresy and its immediate aftermath, when the light of the Great Crusade guttered and the young Imperium came within a hair's breadth of annihilation. It was an age defined by fratricide, by primarch turned against primarch and Legion against Legion, and by the intrusion of the warp's horrors into the material realm.
The term evokes more than a single war. It marks the death of an ideal — the rational, enlightened dominion the Emperor had sought to build — and its replacement by fear, dogma, and grief. Communication failed as warp storms raged; whole sectors fell silent; and the certainties that had bound humanity's reconquest of the stars dissolved into suspicion and slaughter.
Though the age is reckoned to have ended with Horus's defeat, its darkness never truly lifted. The Imperium that survived was a maimed and haunted thing, its god-Emperor a corpse upon the Golden Throne and its people taught to dread the very future they had once been promised. In the long memory of mankind, the Age of Darkness stands as the hinge on which hope turned to horror, and the shadow it cast reaches across all the millennia that followed.