Decline and Fall of the Five Hundred Worlds

The History of the Decline and Fall of the Five Hundred Worlds is a historical treatise in six volumes that chronicles the diminishment and eventual dismemberment of the Realm of Ultramar during the Horus Heresy and the Great Scouring. It was written by the renowned historian Edward Gibonius between 622.M34 and 701.M34. Gibonius published both a High Gothic version and a Low Gothic version of the text, with notable difference between the two. There are rumors of a third version which covered topics too delicate to discuss, even in High Gothic. These rumors, while unsubstantiated, are all seemingly in agreement that, if such a version ever existed, all surviving copies are now in the hands of the Inquisition.

Low Gothic Version
The Low Gothic version relates the fate of Ultramar in manner consistent with the history known to the common folk of the Imperium. It relates how the Arch-Traitor, General Horus, led his mortal followers in Rebellion against the God Emperor and did battle with the Emperor's loyal servants, including the vaunted and incorruptible Primarchs and Space Marines.

The history begins with the betrayal at Calth, where a human General known only as The Servant of the Arch-Traitor launched a dishonorable sneak attack upon the Ultramarines Primarch, Roboute Guilliman and his friend and spiritual advisor, Lorgar Aurelian. Throughout the Low Gothic version, the Ultramarines are portrayed as a Chapter sized force, although they are referred to interchangeably as the XIII Legion. Guilliman's sons are portrayed as thoroughly heroic and key to every Imperial victory in the region, but desperately overstretched trying to defend the five hundred worlds with a force of no more than 1000 Marines.

Gibonius goes on to cover the Shadow Crusade, in which the forces of the Arch-Traitor raided and burned their way through half the five hundred worlds of Ultramar to keep the Ultramarines tied down and unable to come to the Emperor's assistance.

It was, according to Gibonius, the arrival of Guilliman's brothers Sanguinius and Lion El'Jonson that ended the Shadow Crusade and allowed aid to be dispatched to Terra. The account goes into great detail when covering the event whereby the three Primarchs sought out the wisdom of the holyman, Lorgar Aurelian, as to which brother should be in the lead enroute to Terra. Logar convinced the three that, while the Lion had the most Marines left to him, and Gulliman was the greatest tactician, it should be Sanguinius in the lead, for he was the most Holy.

So it was that Sanguinius, accompanied by Logar, was the only one able to stand at the God Emperor's side when he confronted the Arch-Traitor and so it was that the Angel Sanguinius fell and was martyred. A brief account is given of the God Emperor's battle with Horus, and how the traitor was slain and the God Emperor wounded and installed upon the Golden Throne.

The History concludes with the Great Scouring, as it pertained to the Realm of Ultramar, and an account of how Roboute Guilliman, in an act of pious humility, gave up all but the last dozen of his once five hundred worlds.

High Gothic Version
The High Gothic version of the text is far more honest and reveals many details deemed too dangerous to be shared with the Imperium as a whole. For starters, it is revealed that Horus's followers did in fact include Space Marines, and that the Space Marine Legions we're quite a bit larger than modern Space Marine Chapters.

During the discussion of Calth, the Servant of the Arch-Traitor is named as the Marine Kor Phaeron, Legion Master and overall commander of the VIII Legion. It is also revealed that the relationship between Logar and Guilliman was no more than cordial. The two respected each other, of course, but Gibonius acknowledges that the close personal relationship between the Lord of Ultramar and his Confessor, as portrayed in the Low Gothic version, was a fabrication.

In its portrayal of the Shadow Crusade, the High Gothic version admits to the much greater toll in Imperial lives taken by the traitor forces. There are acknowledgments of Imperial defeats and loyal worlds depopulated. Likewise, the reunion of the three brother Primarchs is represented in a less noble light. The three are shown in heated disagreement, not merely over the order in which they should proceed to Terra, but over the broader strategy for the war, and whether the God Emperor even required their assistance. It is even recorded how the Lion and the Guilliman almost came to blows during their strategy session, with Logar being barely able to convince the two to remain civil.

Finally, following the conclusion of the Heresy, the High Gothic version recounts the division of the Legions into Chapters of no more than 1000 Marines each. In this light, Guilliman's disassembly of the Realm of Ultramar is understandable, less as an act of humility, and more as part of the Lord Commander's broader strategy to decentralize power so that no one would ever again be able to pose the same threat that Horus had.

Forbidden Version
There is no rockcrete evidence that a third Forbidden Version of Gibonius's Decline and Fall was ever written. If it did exist, there is no agreement as to what differentiated it from the High Gothic Version.

However, certain rumors have persisted, in spite of the Inquisition's best efforts to expunge them. The most common, least contradictory versions of those rumors include the following elements:

The Imperial Truth - There is no general agreement upon what the term The Imperial Truth means in this context. However, scholars who believe in the existence of the Forbidden Version speculate that it refers to some form of doctrinal dispute, possibly even a concept declared heretical by the God Emperor himself while he walked among men. How this concept may have been debated by the Ecclesiarchy at the time, or what role it may have played in the broader Heresy is unknowable short of uncovering a genuine copy of the Forbidden Version.

The Great Oath - This rumor maintains that the Primarch Roboute Guilliman swore a great oath during the betrayal at Calth, that has since been concealed from the Imperium at large. What the contents of this oath were, or against whom it was declared are unknown, but it has been suggested that Guilliman's sons maintain knowledge of the oath to this day. While the natural assumption would be that such an oath would sworn against the traitorous Legion Master, Kor Phaeron, the one thing all rumors agree upon was that this was not the case.

The Contingency - In the esoteric, and often borderline heretical, circles in which the possibility of Forbidden Version is discussed, there is often mention of an unknown contingency plan, conceived by loyalist forces. Roboute Guilliman and Lion El'Jonson were arguably the two greatest strategic minds among the Nine Primarchs. If any two were to consider what might be done should the worst happen, should the God Emperor be driven from Holy Terra itself, it would be one or both of these. While no one can know for certain, it is worth noting that the Lion's homeworld of Caliban was destroyed directly after the conclusion of the Horus Heresy. There is a possibility, however remote, that Caliban was meant to be a second Throne World to which the God Emperor could retreat to, should Terra fall. If so, the world's destruction may have been undertaken as the only means available to cover up the existence of this unremembered empire.