"An Outside Context Problem was the sort of thing most civilizations encountered just once, and which they tended to encounter rather in the same way a sentence encountered a full stop."
- – Iain M. Banks, summing up the end result of Star Trek type societies like The Interex encountering the Imperium quite nicely
The Interex (both the demonym and the empire, used like "the Imperium") were basically what the Tau think they are. The word "Interex" derives from the real life word "Interrex"; being the title of the leader in the ancient Roman Kingdom that takes control during an Interregnum, with the title literally meaning "Between Kings". Which considering their eventual fate is just as hack a name as Ferrus fucking Manus.
The Interex, probably the closest thing we can glimpse about Human Civilization during the period of enlightenment known as the Dark Age of Technology, was made up of technologically advanced humans, but also alien races like the kinebrach, which were like space gorillas. It is notable to mention that the Interex learned about Chaos from, and were tutored by, the Eldar. Perhaps Eldar arrogance only extended to those humans who lost technology. Interex ships were bigger and more advanced than those of the Imperium, and the empire also made use of AI and robots (that could additionally be used to augment their troops). While they were very powerful and capable of wiping out hostile xenos, they felt it was better to just downgrade their technology, eliminate space tech and scale the populace down to one planet, surrounded by warning satellites. They were aware of and fought against Chaos, which they called Kaos, unlike the Imperium at the time, which officially denied the existence of daemons and spirits. The Interex language was augmented/accompanied by some kind of music based on universal math called the aria, which made them cool and not prissy. Their warriors could ride on four-legged robots to form a centaur-like warrior that fought with laser arrows that could easily penetrate power armor. Regular Interex warriors could just about take an Astartes in single combat, mostly because the Astartes underestimated them and because the Interex's fighting styles were almost unrecognizable to them. They felt that war was bad though, and this idealism would lead to their replacement by the Imperium as prime galactic power. After taking the Interex seriously, the Astartes quickly kicked their asses. For all the tech it lacks, the Imperium still possessed outstanding material science. As a result, bolters and chainswords were and are really dangerous, even to the Interex.
In the first Horus Heresy book, Horus meets them and decides to actually negotiate with them, even though he knows the Interex makes use of the robots and AI, which are abolished by the Emperor (thanks to that whole Men of Iron thing that fucked over everyone before), and because he feels that the galaxy is too grimdark and the Imperium purges things too often. When it seems that everything is going well, and the Imperium might just not suck if Horus thinks like this, Erebus steals a kinebrach sword called an Anathame, THE MURDER SWORD (it's related to Nurgle), from a war museum called the Hall of Devices, blows the museum up, and starts a war. Horus is scratched and sulks off to feel bad, while the Interex are mentioned to have been purged in an offhand mention in the next book.
The Interex makes an excellent contrast against the Imperium in several ways. Firstly, there's the comparison between them (or what little we know of them) and the Imperium both pre-Heresy (already showing the rot beneath its gilt facade) and post-Heresy (techno-barbaric theocratic fascist feudalistic hellhole). In comparison, the Interex made the truth about the denizens of the Warp plain and simple to all its people, ensuring every member of their culture knew exactly what the Chaos Gods were after and the dangers they posed. The result of this education? Chaos basically couldn't get it's hooks into any Interex: they were so powerless against them they had to manipulate the Space Marine Legions into doing their dirty work for them.
A legit point to argue is that their lack of militarization would have led to catastrophe if the Tyranids or Necrons were to show up on their doorstep, or even one of the Ork Empires of the time (as one of them required the efforts of multiple legions to topple, whereas the Interex were wiped out by a single legion). The Interex clearly had the ability to defend themselves against smaller groups of hostile xenos (as shown by their defeat of the megarachnids), but they just weren't geared to fight a galaxy-conquering force, or even a force able to control more than one star system. It is still intriguing to imagine what might have happened if Erebus' plan was foiled, or what other Legions might have made of the Interex if they'd come across them before Luna Wolves did. Like, say, the idealistic Ultramarines, or the technophilic Iron Hands. Or even better: The Thousand Sons. That would have been hilarious (from a safe distance, anyway).
But were the Interex truly immune to Chaos?
Let us consider for a moment a few points, first off the Interex had received help from other alien races with some experience on dealing with Chaos, indeed, this is an advantage which may not have been wielded by many human civilizations in the setting. Secondly there is the simple fact that the Interex was a post-scarcity economy, this alone means that next to no one would like to attempt to deal with enemy forces out of physical desperation, which is often one of the major reasons why Chaos takes hold in the poverty-stricken worlds of the Imperium. Like in real life, the vast majority of people will not feel any interest in joining the enemy, or a strange cult when things go well.
Thirdly, the Interex was a collection of small territories when compared with the Imperium, so it was far easier for them to manage their population and bring education to the masses while keeping a better control over them, and if this sounds familiar to you, great, you are paying attention.
Finally, Chaos really didn't need to corrupt anyone to take hold over the Interex - all it needed was to use someone to wipe them out on their behalf. It also might have been possible for Chaos to corrupt some of them eventually. As Radical Inquisitors have shown, being fully educated on what Chaos is and its dangers doesn't prevent you from falling to it, and in some cases it actually helps the fall. In the Interex's case, their awareness of Chaos led them into a state of complacency that allowed the Ruinous Powers to destroy them with the help of outside agents.
Ultimately it can be recognized that the Interex looked like a possible path for a better future for mankind, but all it takes to fuck it up is one guy willing to take advantage of a bad decision, like choosing to keep and study a Daemon Weapon (or any Chaos artifact really) instead of destroying it.
The kinebrach were a race of space gorillas that the Interex conquered and assimilated. Demilitarized when not under human escort, they were civilized and cooperative enough to not be nuked back to the stone age like the megarachnids.
The Interex effectively merged their civilization with alien civilizations, both robbing those aliens of their own species' identity and accomplishments and making the humans vulnerable to becoming little more than "those guys who make cool stuff for everyone but otherwise have no power". We do not know how the member races got along, but in the Interex, the humans were clearly dominant.
The Interex knew that the kinebrach dealt with Chaos, and were consumed by lethargy and stagnation. The anathame is also called the kinebrach sword because it was made by the Kinebrach. Given how their sword had a magical disease on it, they were clearly a race claimed by Nurgle. In return for their servitude, the Interex saved the kinebrach from being fully consumed. You know that Chaos got them back for that.