The Horus Heresy was fought across the galaxy for more than seven years, beginning with the terrible Istvaan III Atrocity where Horus cleansed four of the Traitor Legions under his command of their remaining Loyalist elements and the Drop Site Massacre of Istvaan V, where Horus and his Traitor Legions nearly decimated three entire Loyalist Space Marine Legions through the most basest of treacheries. Horus, sought to achieve a swift and decisive victory over the Emperor after pledging his soul to the service of the Ruinous Powers of the Warp, leading his Traitor forces directly to Terra over the next seven standard years, slaughtering tens of billions of people and destroying much of what the Emperor had built over the last two centuries
. In a final decisive battle between the Emperor and Horus at the end of the great Battle
of Terra, the Warmaster was slain, leading to the end of the rebellion when the Forces of
Chaos naturally began to turn on one another without Horus' ambition, titanic charisma
and influence to keep them united in pursuit of a single goal. Over the next several years,
the Traitor Legions and their Chaotic allies slowly withdrew from Imoperial space,
eventually fleeing Imperial pursuit into the great Warp rift in the Segmentum Obscurus
known as the Eye of Terror.
However, the Master of Mankind was Himself mortally wounded during the battle, which took place on Horus' Chaos-twisted Battle Barge, the Vengeful Spirit, in orbit above Terra. The Emperor's body was recovered by the Primarch Rogal Dorn of the Imperial Fists Legion and according to His last instructions, the Emperor was placed on the life-preserving Golden Throne, an arcane artefact from the Dark Age of Technology that served as both a psychic amplifier and a potent cybernetic life-support system that the Emperor had originally intended to use to create a portal into the Webway. For over 10,000 years now, the Emperor has remained immobile on the Golden Throne. Though physically a broken, dying carcass incapable of movement and unable to communicate normally with the outside world save through the rare telepathic contact and the Emperor's Tarot, the Emperor's psychic will, almost omnipotent, extends through the Immaterium across the million worlds of the Imperium. It produces the psychic beacon of the Astronomican that is used by all Imperial starships to travel through the Warp, soul-binds weaker psychic humans to make them useful Sanctioned Psykers of the Imperium while leeching the lives from other psykers to sustain His psychic presence, and struggles against the encroaching daemons of the Warp, protecting Mankind from their realspace predations. In all this, the Emperor endures the constant agony of His dying body only through a sheer exercise of will, and the sustenance that the life forces of the sacrificed psykers provide. Yet the masses of humanity worship the Emperor as both a God and their only saviour. It is an article of faith in the Imperial Cult that against all the threats faced by the human race, only the Emperor protects...