The Interex were a highly advanced interstellar human civilisation that existed across some thirty star systems at the time of the Great Crusade in the 31st Millennium. The Interex, unlike the Imperium of Man, had decided that the best way to deal with intelligent alien species was not to exterminate them or make war upon them, but to try and coexist with them. In fact, the Interex had become the close allies and nominal overlords of a simian alien species called the Kinebrach who they had integrated into their own society. The Interex ultimately became enemies of the Imperium after hostilities between the two human civilizations began when the Interex accused a diplomatic party of Luna Wolves led by the Warmaster Horus with breaking into their Hall of Devices, a museum on the Interex world of Xenobia dedicated to weaponry, and stealing a prized Kinebrach blade called an Anathame that was known to have links to Chaos. The Interex believed that the Imperium was allied to Chaos and never knew that in fact the weapon had been stolen by First Chaplain Erebus of the Word Bearers Legion, who had been with Horus' diplomatic party. Unknown to Horus, Erebus and the Word Bearers Legion had already turned to the worship of the Ruinous Powers of Chaos. The powers of the Warp had informed Erebus and his Primarch Lorgar that if they wished to bring Horus over to the side of Chaos, then they would need the Kinebrach Anathame. The Interex civilisation was ultimately wiped out by the Luna Wolves following the commencement of hostilities between the two human factions on the night Erebus stole the Anathame.
The Interex maintained a close alliance with alien races such as the Kinebrach. They were first encountered by the Luna Wolves during the Great Crusade following that Legion's war against the world of the Megarachnids called Murder by the Imperium, which was actually a prison planet for that vicious alien species. The Megarachnids had been imprisoned on Murder by the Interex after being defeated in a terrible war and deprived of all their space-faring technologies. The technology and science of the Interex was in many ways more advanced than even that of the Imperium's, but was not as strongly focused on the waging of war.
Devoted to fighting Chaos, (which they referred to as "Kaos"), they regarded the Imperium's 63rd Expeditionary Fleet skeptically, as they thought the Imperium might be allies of Chaos. The Interex's government was in diplomatic talks with Horus and his peers when Erebus of the Word Bearers, a Space Marine Chaplain tainted by the powers of Chaos Undivided, infiltrated an Interex museum of technology called the Hall of Devices housing alien artefacts and weapons, stole the Kinebrach Anathame and rigged the building to explode, which only affirmed the Interex's initial fears about the Imperial forces and inaugurated an unfortunate war between the two human cultures. Fighting broke out between the Space Marines and Interex forces on their world of Xenobia until Horus was successfully evacuated off-planet. It should be noted that the Interex warriors were quite a match for the Luna Wolves Space Marines under the Warmaster Horus' command. The Interex were later destroyed by the Luna Wolves as part of the Imperium of Man's Great Crusade following the tragic misunderstanding on Xenobia.
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 platform used by Interex warriors. They would walk into the base, and form a warrior who looked much like the mythological representation of a centaur with increased mobility. They also used a type of bow or crossbow that Horus' Space Marines wrote off as decorative parade weapons, until a skirmish revealed that they could emit bolts of laser light fully capable of punching through even a Space Marine's armour. The Interex also had automated drones throughout their society that were akin to advanced artificial intelligences as opposed to the Imperium's less humanitarian use of cybernetic Servitors. However, the Mechanicum chose to use Servitors because of humanity's prior experience with the Iron Men of the Dark Age of Technology. Perhaps one of the foremost commandments of the Adeptus Mechanicus' Cult of the Machine was that no truly artificial intelligences could be created, so instead they used partially organic, mind-wiped cyborgs to serve as laborers and soldiers.