The Great Rift, also known as the Cicatrix Maledictum in High Gothic, is a tear in reality and a raging, galaxy-wide Warp Storm that has essentially rent the territory of the Imperium of Man in half. The rift is known by the other sentient cultures of the galaxy by various names, including the "Crimson Path," the "Mouth of Ruin," the "Warpscar," the "Dathedian," "Gork's Grin" and a thousand other names besides. Even before the fall of Cadia, the Immaterium had been growing ever more turbulent. Travel and communication suffered increasing disturbances as the events of Abaddon the Despoiler's 13th Black Crusade unfolded in 999.M41. Eventually, the Materium could take no more, and a titanic Warp rift tore across the galaxy, spreading from the Eye of Terror to the Hadex Anomaly on the Eastern Fringe. To those on the Terran side of the rift, it is a tainted scar stretching across the sky. To those unfortunates on the far side, in the region now named the Imperium Nihilus, the "Dark Imperium," it is something much worse -- the very gates of Hell.
No one fully understands the origins of the Great Rift, though there are many theories: the breach of the Cadian Gate during the Despoiler's recent 13th Black Crusade, the sorcery of the Daemon Primarch Magnus the Red during the Thousand Sons' invasion of the Fenris System, catastrophe in the Webway, mass bloodshed and fire in the Damocles Gulf -- all may have caused or contributed to it. Its emergence was a literal galaxy-shattering event, which threw the Imperium of Man into chaos and ushered in new wars across nearly every world in the Emperor's domain. So powerful and far-reaching was this Warp rift that the very laws of physics fray at its edges as the inconsistencies of temporal fluctuations, once largely localised to larger Warp Storms such as the Eye of Terror, spread across the galaxy. Some worlds felt standard centuries go by in an instant while others were all but frozen in time, and still others have suffered constant temporal shifts.
On the far side of the galaxy-spanning Warp rift from Holy Terra, things have quite literally gone to Hell. There, the light of the Astronomican is obscured behind a psychic maelstrom of nightmares and the entire region has been dubbed the Imperium Nihilus, or the Dark Imperium. Amongst the besieged Imperial worlds in this Dark Imperium are many Space Marine Chapter Planets, including Baal, home of the Blood Angels. Things were already looking grim for the sons of Sanguinius, with a Tyranid Hive Fleet closing in on them, and now it looks even bleaker. Cut off from the light of Terra, the Blood Angels and their Successors stand unbowed against the might of Hive Fleet Leviathan, hoping they can withstand the storm.
In response to the emerging Forces of Chaos throughout the galaxy, the recently resurrected Roboute Guilliman, Primarch of the Ultramarines, and now Lord Commander of the Imperium and its vast armies, has launched his Indomitus Crusade. Gathering his new armada, along with elements of the Adeptus Custodes, a small contingent of the Silent Sisterhood, and a vast war host of Primaris Space Marines from many newly founded Chapters, the Primarch has set a winding course. Strike forces from over a dozen pre-existing Chapters of Space Marines, led by the Imperial Fists, have joined the Indomitus Crusade Fleet. Thus began many new legends as Guilliman travelled to aid beleaguered Imperial planets, breaking sieges and sweeping away invaders to bring hope back to the desperate defenders. It was not long before word began to spread, as those worlds that could still receive astropathic messages hailed the return of a hero out of myth. Once more, one of the demigods of the past fought for the Imperium of Mankind.
But the scale of Guilliman's task is monumental. Following the emergence of the Cicatrix Maledictum, no world in the Imperium has been untouched by war, and there are many thousands of planets that cry out for aid and reinforcement.
- Warhammer Community: The Indomitus Crusade & The Dark Imperium
- Warhammer Community - New Warhammer 40,000: The Great Rift
- Warhammer Community - New Warhammer 40,000: The Galaxy Map