The Cadian Gate is an area of calm space leading into and out of the massive Warp Rift called the Eye of Terror. It is located to the galactic southeast of the Eye in the Segmentum Obscurus, and is the only predictably stable way out of that maelstrom of Warp energy. It is therefore a site of frequent raids by the Forces of Chaos seeking to break through the Imperial defences in the region so that they can invade the larger Imperium of Man.
Several Black Crusades have emerged from the Cadian Gate, including the most recent campaign (13th Black Crusade) led by the Chaos Lord Abaddon the Despoiler. This has led to its continuous monitoring by the Imperium and the stationing of several forces of Imperial Guard in the closest star system, Cadia. Abaddon the Despoiler is the only Chaos Lord to date to gain a foothold on the Fortress World of Cadia during the 13th Black Crusade of 999.M41 in which minor Chaotic victories resulted in the capture of large swathes of the planet from its valiant defenders.
The Cadian Gate is not the only stable exit from the Eye as the Arx Gap offers another way to exit the Warp Rift, although it is somewhat less reliable. It was from the Arx Gap that Abaddon began the Gothic War (the 12th Black Crusade) by raiding the research and monitoring station located there in 139.M41. The Arx Gap bypasses the powerful defences of the Cadian Gate, thus making it a more attractive location from which the Forces of Chaos can stage their assaults on the Imperium, but it is more prone to Warp turbulence.
When the defeated Traitor forces collapsed with the death of Horus and fled in disarray from Terra, some Loyalist forces rallied and gave chase, but most remained on Terra to consolidate their great victory. Many rebels were put to the sword, but the majority of the Traitor Legions escaped to the realm of space known as the Eye of Terror, that region where reality and insanity collide and the raw energy of the Warp pours into realspace in a swirling maelstrom. There the Chaos Gods rule over uncounted planets, all warped to their own evil aspects, and this is where the Traitor Legions found refuge from their pursuers, isolated from the galaxy by powerful Warp Storms.
Every world within the Eye of Terror is a Daemon World, warped and twisted by the whims of the Gods of Chaos and the Daemon Princes who rule them. The Traitor Legions regrouped and nurtured their hatred, planning for the day when they would wreak a terrible vengeance on those who had defied them. Within the Eye, time flows very differently than in realspace, sometimes faster, often slower. Those same Traitors who fought on Terra at the Siege of the Imperial Palace in the early 31st Millennium still fight in the service of the Ruinous Powers. They fight against each other to prove their supremacy, and against the forces of the Imperium when the Warp Storms calm long enough to allow them to rampage into Imperial space. The Imperial sectors surrounding the Eye of Terror are heavily militarised to resist these Chaotic invasions when they come, and none more so than Cadia , the Fortress World that stands at the mouth of the one stable route leading from the Eye of Terror, the Cadian Gate. Twenty separate Chapters of the Adeptus Astartes were eventually Founded to specifically aid in the defence of the worlds surrounding the Eye of Terror and the Cadian Gate and they are known as the Astartes Praeses.
Aftermath of the Fall of Cadia
At the culmination of Abaddon the Despoiler's 13th Black Crusade, the Cicatrix Maledictum had all but consumed Cadia. With the great Cadian Pylons toppled by Abaddon's conquering hordes and the colossal Warp Storm of the Eye of Terror no longer held at bay, the dread powers of Chaos ravaged the planet beyond recovery. And still the neighbouring worlds and star systems were defiant.
Had Cadia not been famous for holding so long against the odds, it is likely the other planets in the system would have capitulated or given up almost immediately. No normal world could stand in the face of the horrendous, sanity-devouring armies that descended upon the Cadian Gate in such terrifying measure. However, the wider systems of Cadia, Belis Corona, and Agripinaa stood fast.
Perhaps, just as a veteran can lead lesser men to deeds of great heroism, Cadia had inspired those worlds around it to defend every nation and city to the last bullet. Perhaps Cadia's surviving soldiery lent strength through their hunger for retribution. Perhaps those battered by the storm simply fought for survival. Whatever the reason, the defenders of the Cadian Gate resolved to uphold the virtues of its lost lynchpin world come what may.
In the confusion of outright war, the hordes of Chaos began to turn upon themselves. Though they were ascendant, their ultimate conquest had yet to be clinched. Rival warlords, both mortal and daemonic, clashed over the spoils of victories not yet won. The Imperial defenders were quick to capitalise on each strategic misstep,for many were veterans of the wars upon Cadia, and they knew how to goad a fractious enemy into overextending its reach. From dissolution came destruction. Solar day by solar day the Imperial armies clawed back a semblance of control. Soon the war zone was in contention once more, the dread stranglehold of Chaos loosened by the sheer determination of the Astra Militarum and the vengeful fury of the Space Marines. Each new dawn was greeted by a scattering of ships from the Great Exodus, those fortunate souls who had braved the Empyrean tempests and lived to tell of it. Again and again the fighting escalated, the fires of war that had burned Cadia to cinders roaring to life anew upon the other worlds of the Cadian System, as well as those of the Belis Corona and Agripinaa Systems.
Many notable Imperial Navy battlefleets protect the surrounding systems of the Cadian Gate by constant patrolling the space lanes around the Eye of Terror, constantly vigilant for Chaos raiders or another approaching Black Crusade.
- Codex: Eye of Terror (3rd Edition), pp. 4-5
- Warhammer 40,000: Rulebook (8th Edition), pg. 161