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 after ca. 999.M41. 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 in the final years of the 41st Millennium. 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.
With the coming of the Great Rift, the galaxy has been riven with Warp Storms of unprecedented fury. Some permanently circle the Great Rift, while others travel in an almost sentient fashion all over the galaxy, obeying no known rules of solar current or winds. Such storms cut worlds off from the light of the Emperor in the form of the Astronomican, and countless civilisations caught in their shadow have been terrorised, enslaved, or altogether destroyed by the daemon legions, or otherwise suffer from mutation and corruption. There can be no doubt that a dark new era is dawning...
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, the birth of Ynnead, the Aeldari God of the Dead, mass bloodshed and fire in the Damocles Gulf, the resurrection of the Primarch Roboute Guilliman -- 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. The result has been an update of the Imperial Calendar ordered by Roboute Guilliman to both correct the growing inaccuracy of the prior chronology and provide a new dating system that uses the opening of the Great Rift as the point in time from which all further chronological calculations are made.
On the far side of the galaxy-spanning Warp rift from Holy Terra and the half of the galaxy called the Imperium Sanctus, 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. Many of those planets in the vicinity of the Great Rift have disappeared entirely, or been so corrupted that they are now labelled as Daemon Worlds.
Amongst the besieged Imperial worlds in this Dark Imperium were 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. Cut off from the light of Terra, the Blood Angels and their Successors stood unbowed against the might of Hive Fleet Leviathan during the Devastation of Baal, hoping they could withstand the storm, saved in the end only by the arrival of aid unlooked for from both the servants of the Emperor as well as His vilest foes.
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, 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 set a winding course.
Strike forces from over a dozen pre-existing Chapters of Space Marines, led by the Imperial Fists, 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 remains monumental, even in the wake of the conclusion of the Crusade and his successful repulsion of the forces of the Plague God Nurgle from his home Realm of Ultramar during the Plague Wars. 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 still cry out for aid and reinforcement.
The vast majority of the Greenskin race knew nothing of the events leading up to the birth of the Great Rift, or its true cause in the struggle of the Imperium of Man against Chaos. All they saw was an almighty rent tearing across the vastness of space and consuming everything it touched.
The Orks have only two gods: Gork, who is brutal but kunnin', and Mork, who is kunnin' but brutal. To the Greenskins, it was obvious that the Great Rift was in fact the leering gob of Gork, opening wide to swallow the stars. Admittedly, a prodigious number of Renegades, daemons and other creatures of Chaos were issuing forth from this sprawling astronomical phenomenon, but as the smartest Orks of each tribe were quick to point out, Gork obviously did not want to swallow that sort of unnatural filth.
Instead, he was vomiting up the Chaos worshippers so that the Orks could fight them and win. Equally, those Ork tribes whose worlds were consumed by the spreading of the rift bore their god no ill will. Clearly, Gork had decided that those tribes were "goin' a bit soft", and so had eaten their planets in order to propel them out into space and send them on the warpath.
- Codex Adeptus Astartes - Deathwatch (8th Edition), pp. 22-23
- Codex Adeptus Astartes - Grey Knights (8th Edition), pp. 18-19
- Codex Adeptus Astartes - Space Marines (8th Edition), pp. 16-17
- Codex: Adeptus Mechanicus (8th Edition), pp. 12-13
- Codex: Astra Militarum (8th Edition), pp. 16-17
- Codex: Chaos Daemons (8th Edition), pp. 6-9, 26-27
- Codex: Drukhari (8th Edition), pp. 42-43
- Codex: Genestealer Cults (8th Edition), pp. 34-35
- Codex Heretic Astartes - Chaos Space Marines (8th Edition), pp. 20-21
- Codex Heretic Astartes - Death Guard (8th Edition), pp. 18-19
- Codex Heretic Astartes - Thousand Sons (8th Edition), pp. 24-25
- Codex: Imperial Knights (8th Edition), pp. 12-13
- Codex: Orks (8th Edition), pp. 7, 26-27
- Codex: Necrons (8th Edition), pp. 14-15
- Codex: Tyranids (8th Edition), pp. 40-41
- Imperium Nihilus - Vigilus Defiant (8th Edition), pg. 10
- 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
- Warhammer 40,000 Timeline - M41: The Gathering Storm