Interex

The Interex (both the demonym and the empire, used like "the Imperium") were basically what the Tau think they are.

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. 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.

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, 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), blows up a war museum called the Hall of Devices, 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 hellhole). More obviously, there's the whole comparison of their own attitude towards Kaos/Chaos and the Imperium's attitude towards the same, which highlights just how useless both the Imperial Truth and the Imperial Cult are.

Especially the latter: for all it's fans defend it, the truth is that countless Imperials are lost to Chaos by ignorance or forbiddance-induced curiosity every day. Humans don't know anything about Chaos, or else know just enough to be intrigued, but not enough to know the full dangers. Even the Space Marines, whose brains are much more "muffled" to most human desires and are brainwashed with Imperial Cult propaganda as a part of their creation, go rogue all too often. In comparison, the Interex made the truth about the denizens of the Warp plain and simple to all it's 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.

If not for the God-Emperor of Mankind and his totalitarian dreams for his Imperium, perhaps it might have been the Interex who would have led the reunification of humanity. Perhaps then, the far future would not have been grimdark, but noblebright. Which just makes the setting of 40k even more grimdark. It's also worth wondering what other Legions might have made of the Interex if they'd come across them before Luna Wolves did. Like, say, the Warp-skeptical, riding-loving White Scars. Or even better: The Thousand Sons.