Interex

The Interex was basically the Imperium of BrightHammer40k.

The Interex 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. 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, officially denied the existence of deamons. Despite the war like nature of the imperium, its strange the Interex suspected the imperium for being tainted by Kaos because Horus brought remembrancers, and Iterators into were the outpost they were having negotiations at. Remembrancers arn't known for their discipline or following orders, and so even if they were told not to bring up the Imperial Truth, one might have anyways. One of the primary goal of Iterators is the abolishing religion and superstition including the belief in spirits, magic, and deamons. The Interex language is also some kind of music based on some universal math, 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 were almost as good as an Astartes. They felt that war was bad though, which it is.

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 museum, 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 Space Greeks got ass raped by Space Romans. Grimdark.