The Cadian Gate is an area of calm space leading in and out of the Eye of Terror. It is located to the galactic south-east of the Eye, and is the only predictably stable way out. It is therefore a site of frequent raids by the forces of Chaos.
Several Black Crusades have emerged from here, including the most recent, the 13th led by Abaddon the Despoiler. This has led to its continuous monitoring and the stationing of several forces of Imperial Guard in the closest system, Cadia.
It is not the only stable exit from the Eye, the Arx gap is another way to get out, although it is somewhat less stable. It was from here that Abaddon began the Gothic War by raiding the research and monitoring station here in 139.M41. It bypasses the Cadian Gate, as such making it a more attractive place to attack from.
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.