The Noctis Aeterna, the Blackness, was a period of galaxy-wide darkness and terror that fell upon the Imperium when the Great Rift tore open in the wake of the fall of Cadia. As the warp erupted into reality, the light of the Astronomican, the psychic beacon by which all Imperial ships navigate the immaterium, was smothered across vast reaches of space.
Without that guiding light, warp travel became all but impossible, and interstellar communication through the astropathic choirs failed. Countless worlds were plunged into sudden, absolute isolation, cut off from reinforcement, supply, and even news of the wider Imperium. In that long night, planets fell prey to daemonic incursion, rebellion, and despair, with no hope of rescue and no way to call for aid. Entire sectors simply vanished from Imperial knowledge, their fates unknown.
The Noctis Aeterna was less a single battle than an all-encompassing crucible of dread that accompanied the birth of the Great Rift. For those who endured it, survival meant standing alone against every horror the galaxy could conjure, trusting that the darkness might one day lift. When the light of the Astronomican at last began to pierce the gloom in places, the Imperium that emerged was transformed, scarred by isolation and hardened by the knowledge that the age of easy passage between the stars had ended forever.