The Iterators were a body of the early Imperium dedicated to speech, diplomacy, and communications. These were the guys that helped the Imperium absorb worlds via diplomatic means rather than warfare. There were quite a few worlds taken in this way in the Great Crusade, but in the Grimdarkness of the far future there is only war. So while they accomplished quite a bit for the Imperium, they weren't considered relevant enough or exciting enough for the focus of the Horus Heresy series to even be close to these guys. Their job was also to justify to themselves and to the conquered worlds that everything the Imperium represented was utterly good. Honestly, iterators were the masters of propaganda.
They were largely dedicated to using nonviolent means, which meant that most of them were probably killed off or, if loyal, absorbed into other administrative branches, during the Horus Heresy due to that period's inherent nature to Rip and Tear everything and everyone involved. However, some did follow Horus, who saw the value of keeping some diplomats close at hand, to sway undecided worlds which didn't require the personal touch of a subversive agent, or where and invasion would not be worth the resource investment in the long run.
In the years following the Heresy, they were eventually disbanded since, you know: war never changes, though this was probably more due to circumstances pulling the iterators in different directions, rather than a blatantly outrageous and retarded decree that had the Iterators disbanded. Many see the Commisarriat as a martial counterpart of the Iterators; reinforcing morale and obedience within the Imperial Guard, which later naturally evolved into doing so with bolter rounds due to the nature of their work environment. Many also see the Ecclesiarchy having their roots with the Iterators, especially with the influence of Kyril Sindermann, an Iterator attached to the pre-Heresy Luna Wolves legion and a chaperone to the first Imperial Saint. This helps explain the Ecclesiarchy's role within the Imperium as simultaneously being charismatic demagogues and pompous assholes.
During the Great Crusade, and possibly afterwords, the Iterators found themselves in a shrinking role on the galactic stage, as warfare and superfluous genocide became standard means to an end. The role of professional diplomats and ambassadors became increasingly viewed as nonessential roles within the post-Heresy and post-Emperor Imperium of Man.
Notable Members of the Iterators
- Kyril Sindermann - Primary iterator of the Imperial Truth, until he saw a space marine turn into a Flamer of Tzeentch, and then later became a founding member of the Imperial Church and the Inquisition.