Interex

The Interex was 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 they actually used AI and robots (that could be also used to augment their troops). While they were capable of wiping out hostile xenos, they felt it was better to just scale them down to one planet surrounded by warning satellites and make the hostile race incapable of spaceflight. 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. The Interex language is augmented/accompanied by some kind of music based on universal math called the aria, but they are not prissy. Their warriors could combine with four-legged robots to form a centaur-like warrior that fought with laser arrows. These 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 unrecognisable 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, 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 under Horus, Erebus steals a kinebrach sword called an Anathame (which was 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.

tl;dr Star Trek got ass raped by Space Nazis. Grimdark.

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 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 its 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 literally rewired to strip them of most human desires and to induce fanatical loyalty to the Imperial Cult as part of their creation, go rogue all the time. In comparison, the Interex made the truth 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 its hooks in 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 dreams of the Imperium, perhaps it might have been the Interex who led the reunification of humanity. Perhaps, then, the far future would not have been grimdark, but noblebright. Which just helps make the setting even more grimdark. It's also worth wondering what other Legions might have made of the Interex if they'd come across them before Horus & Sons. Like, say, the Warp-sceptical, riding-loving White Scars.