The Interex maintained a close alliance with alien races such as the Kinebrach. They were encountered by the Luna Wolves during the Great Crusade. The technology of the Interex was in many ways more advanced than that of the Imperium's, but was not as strongly focused on the waging of war. Devoted to fighting chaos, (they referred to it as 'Kaos') they regarded the 63rd expedition sceptically, as they thought they might be tainted by it. They were in talks with Horus and his peers when a marine (First Chaplain Erebus of the Word Bearers) tainted by the powers of Chaos infiltrated the Hall of Devices, the Interex museum of artefacts and weapons, sabotaged the building to explode and stole a valuable and dangerous Kinebrach sword known as an Anathame. Fights broke out between Space Marines and Interex forces until Warmaster Horus was successfully evacuated off planet.
It should be noted that the Interex warriors were quite a match for the Space Marines under Horus' Command.
As Horus halted his attacks on the Interex, and the Heresy began shortly after. The Interex are confirmed to have been destroyed in an Imperial campaign before the beginning of the war on the Auretian Technocracy.1
The Interex had incredibly advanced technology available to them. At first glance, many of Horus' warriors believed the Interex to be aliens in the shape of centaurs. Later it was found the centaur base was a form of mobile fighting platforms for Interex warriors. They would walk into the base, and form a centaur looking warrior with increased mobility. They also used a type of bow or crossbow that the warriors of Horus wrote off as decorative parade weapons, until a skirmish revealed that they could emit bolts of light fully capable of punching through Space Marine armor.
The Interex also had automated drones throughout their society that were akin to Advanced AI as opposed to the Imperiums more barbaric-like use of servitors (wiped brains serving as CPUs).