Today's topic skips the rules for a day and jumps into the background of the Great Rift.
via Warhammer Community
The Great Rift is a tear in reality that essentially divides the Imperium in half. Its emergence was a literal galaxy-shattering event, which threw the Imperium into chaos and ushered in new wars across nearly every world in the Imperium. So powerful and far was this storm that the very laws of physics fray at the edges as the inconsistencies of time fluctuations, once largely localised to larger storms such as the Eye of Terror, spread across the galaxy. Some worlds felt centuries go by in an instant while others were all but frozen in time, and still others have suffered constant temporal shifts.
None fully understand the origins of the rift – though there are many theories: the breach of the Cadian Gate, the sorcery of a Daemon Primarch, catastrophe in the Webway, mass bloodshed and fire in the Damocles Gulf – all may have caused or contributed to it. The rift is variously known by the cultures of the galaxy as the Crimson Path, the Mouth of Ruin, the Warpscar, the Dathedian, Gork’s Grin and a thousand other names besides. To the Imperium it is the Cicatrix Maledictum, or just the Great Rift. To those on the Terran side, it is a tainted scar across the sky. To those unfortunates on the far side, it is far worse…
While we saw the beginnings of this great tear in the events of the Gathering Storm, in the story of the new Warhammer 40,000, Roboute Guilliman, Lord Commander of the Imperium’s armies, looks out over the ruins of his Father’s empire and sets about its reconquest in a crusade to liberate those worlds embattled by the forces of Chaos and free those beset by xenos predators closing in on the weakened worlds of Mankind.