The Noctis Aeterna

There are defeats that come by the sword, and defeats that come by the smothering of all hope. The Noctis Aeterna was the latter, the Blackness, the terrible span when the Great Rift's opening choked the Astronomican and cast countless worlds into a darkness where no ship could sail and no prayer could reach the Throne. It was less a battle than a drowning, a slow suffocation of the Imperium's light across half the galaxy.

In that blindness, madness bloomed. Cut off from Terra, worlds turned upon themselves; the Chaos Space Marines and the daemon-hosts of the Rift fell upon the isolated and the terrified, and whole systems were lost without ever sending word of their dying. The Adeptus Mechanicus laboured in the dark to keep the machines of empire turning, while the Astra Militarum held vigil on ten thousand walls against enemies that came out of a night no lamp could pierce.

The Noctis Aeterna was not so much won as outlasted, its worst horrors abated only when Guilliman's return kindled the Astronomican anew and the Indomitus fleets carved corridors of light back through the murk. Yet the scars of the Blackness never fully healed, and whole regions remained sundered in shadow. The age that followed was born from this darkness, and carried its dread within it always, a memory of the night when the Imperium nearly went out forever.