Great Rift - History: No one fully understands the origins of the Great Rift, though there are many theories: the breach of the Cadian Gate and the fall of Cadia and its blackstone pylons during Abaddon the Despoiler's 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 partial awakening of Ynnead, the Aeldari god of the dead; the sundering of Craftworld Biel-Tan; the breaking of the Amethal Daemon cage, mass bloodshed and fire between the Imperium and the T'au in the Damocles Gulf; the resurrection of the Primarch Roboute Guilliman -- all and dozens of other events may have caused or contributed to it as the 41st Millennium raced to its bloody conclusion.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 among the Sanguinary Brotherhood 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 fleets. Thus began many new legends as Guilliman and the other fleets 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's first phase 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.