The Great Rift, also called the Cicatrix Maledictum, is a colossal tear in the fabric of reality that split the galaxy from edge to edge, the single greatest catastrophe to befall the Imperium since the Horus Heresy. It erupted in the wake of Abaddon's final assault, a wound in space through which the raw madness of the warp bled unchecked into the material universe.
The Rift's opening was heralded by the fall of Cadia, whose destruction helped unleash warp storms of unprecedented scale. As the tear widened, it severed countless worlds from the guiding light of the Astronomican, plunging them into isolation, madness, and daemonic invasion. Whole sectors were swallowed by the storm, and interstellar travel across the divide became a nightmare of uncertainty.
Most grievously, the Rift cleaved the Imperium itself into two unequal halves. The galactic south retained tenuous contact with Terra, but the northern reaches were cast into a benighted realm known as Imperium Nihilus, cut off from reinforcement and left to fend for itself against the encroaching dark. The Great Rift transformed the very shape of the galaxy, ushering in an age of deeper desperation and endless war. For a fuller account, see the Great Rift.