The Death of Innocence is the name by which later ages remembered the moment the Imperium's hope was broken — the dawning, terrible knowledge that the Emperor's favoured son had betrayed Him and that brother would now murder brother without end. More a wound in the collective memory than a single event, the phrase marks the passing of the confident, enlightened age that the Great Crusade had promised.
For those who lived through it, the death was literal as well as symbolic. It was felt in the ash of the Isstvan System, where loyal warriors were slaughtered by comrades they had trusted with their lives, and in every subsequent betrayal that taught the Imperium to expect treachery from its own. The certainties that had bound humanity's reconquest of the stars — loyalty, brotherhood, a shared and hopeful future — died together in those years.
What replaced them was the grim and suspicious spirit that would define the Imperium ever after. The Death of Innocence names the threshold between two ages: the bright dream of the Emperor's design, and the pitiless, dogmatic dominion born from its ruin. After it, mankind would never again believe itself safe, and never again trust so freely in the promise of a better dawn.