An Iterator was an Imperial rhetorician, propagandist and educator who was a part of the Imperial Corps of Iterators created by the Emperor of Mankind to accompany every Expeditionary Fleet of the Great Crusade during the late 30th and early 31st Millennia. The Corps of Iterators were created by the Emperor specifically to spread and reinforce the atheistic and rationalist Imperial Truth among all the worlds conquered during the Great Crusade and brought into Imperial Compliance. Iterators were present on every Imperial world where they served as educators and propagandists who sought to insinuate the Imperial Truth into every aspect of human life. Iterators paid particular attention to the eradication of any religious or superstitious beliefs among the populace, replacing them with calm, cool rationalism and a strong, almost Victorian belief in scientific progress. Iterators also served as sources of knowledge and inspiration for the Astartes and Imperial Army troops who served in the various Expeditionary Fleets. None could explain the advantages brought by Imperial civilisation or the reasons why the conquest of tens of thousands of new worlds by the Imperium was a necessary requirement for human advancement despite the damage caused to existing human cultures as well as an Iterator.
It was said that the Iterators were selected through a process even more rigorous and scrupulous than the induction mechanisms of the Astartes. "One man in a thousand might become a Legion warrior," an Imperial saying of the Great Crusade went, "but only one in a hundred thousand is fit to be an Iterator." To be an Iterator, a person had to have certain rare gifts that belied simple physical or genetic enhancement, including psychological insight, articulateness, political genius, and a keen intelligence. The latter could be boosted, either cybernetically or pharmaceutically, and a mind could be tutored in history, ethics, politics and rhetoric. A person could be taught what to think, and how to express that line of thought, but he could not be taught how to think, which was the critical specialty of the Iterator. Yet Iterators were not simply schooled in the art of public speaking. They were also trained in both sides of the business of persuasion. Seeded amongst a crowd, Iterators could whip it up into enthusiasm with a few well-timed responses, or equally turn a rabble against the speaker.
In this way, the Iterators were actually the precursors of those Imperial agents who would become the Commissars of the Imperial Guard after the Reformation of the Imperium following the Horus Heresy. Iterators often mingled with audiences, especially on newly conquered Imperial worlds, to bolster the effectiveness of their colleague doing the actual speaking. Most Iterators could be found within the great Archive Chambers of the Astartes Battle Barges that led the Great Crusade's many Expeditionary Fleets. These Archives contained ancient books, hololiths and dataslates left from the ancient past of Terra, some stretching back into the Age of Strife or even the Dark Age of Technology. Iterators were expected to hold tutorials for the Astartes of their fleet to educate them in other aspects of human knowledge for the glorious day when the Great Crusade at last came to its end and peace reigned across the Emperor's vast realm.
The most famous and perhaps skilled of the Iterators was Primary Iterator Kyril Sindermann, who served as the chief Iterator for the Warmaster Horus' 63rd Expeditionary Fleet and dwelled aboard his great flagship, the Vengeful Spirit. Sindermann was a particular favorite of Captain Garviel Loken of the Luna Wolves, with whom he often discussed the weightier matters of philosophy and the reasons why the Emperor had undertaken the Great Crusade to force other human cultures into his Imperium, by diplomacy if possible, but by force if necessary. Sindermann was a dedicated atheist and powerful proponent of the Imperial Truth until the day during the Luna Wolves' conquest of the world designated Sixty-Three-Nineteen when he and several Remembrancers from the 63rd Expedition witnessed the daemonic possession of the Luna Wolves Battle-Brother Xavyer Jubal. To see what could only be described as a daemon called every one of Sindermann's most cherished beliefs into question. He spent months after the incident cooped up in the Vengeful Spirit's Archive Chambers hunting for answers in the knowledge of the human past. He was particularly concerned by one burning question: why would the Emperor have promulgated the atheistic doctrine of the Imperial Truth if he, as humanity's greatest psyker, was keenly aware that such entities existed within the Immaterium?
Sindermann eventually spoke with Euphrati Keeler, the Remembrancer who had taken hololiths and images of Jubal's hideous transformation, but had come to grips with her own confrontation with the daemonic by becoming a devoted follower of the Lectitio Divinitatus, the growing Imperial religious cult that believed in the divinity of the Emperor and ultimately laid the foundation for the Imperial Cult and the Ecclesiarchy. Sindermann attempted to use fragments of text and images taken by Keeler and other Remembrancers to decipher the religious text sacred to the Word Bearers Legion known as the Book of Lorgar which supposedly held the collected wisdom of many dark priesthoods and superstitious faiths across the galaxy and had been created after the Primarch Lorgar had completed his infamous Pilgrimage decades before. While reading a portion of the text aloud from the heretical tome, Sindermann inadvertently summoned a Lesser Daemon from the Warp that began to damage the library stacks of the Archives aboard the Vengeful Spirit until Euphrati Keeler banished the creature back to the Immaterium through the exercise of her extraordinary faith in the Emperor, after which she fell into a coma-like state.
This second encounter with the daemonic broke Sindermann's remaining tenuous belief in the atheism and materialism of the Imperial Truth and forced him to confront the reality that he lived in a universe where spiritual beings did exist and could hunger for the lives and souls of human beings. In such a universe, the only thing that could protect men was the power of other supernatural beings. Thus Sindermann came to exchange the Imperial Truth for a new faith in the divine Emperor as embodied in the teachings of the Lectitio Divinitatus. By this time, the corruption of Chaos had wormed its way deep into the Warmaster Horus and most of his renamed Sons of Horus Legion, and so with the aid of Captain Iacton Qruze of that Legion, Sindermann, the Remembrancer Mersadie Oliton and Euphrati Keeler managed to escape the Venegeful Spirit during the start of the Battle of Istvaan III. They made their way to the Imperial frigate Eisenstein. They were taken in by the Death Guard Loyalists under the command of Battle Captain Nathaniel Garro and escaped from the Istvaan System to Terra after many trials and tribulations to warn the Emperor of his son Horus' great betrayal.
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- The Flight of the Eisenstein (Novel) by James Swallow