The Cadian Gate was the single most strategically vital region in the Imperium for ten thousand years, a narrow and navigable corridor of space that offered the most reliable passage in and out of the Eye of Terror. Through this celestial chokepoint poured the great incursions of Chaos, and so the Imperium fortified it into the mightiest bulwark in the galaxy.
At the heart of this bastion stood the fortress world of Cadia, a planet whose entire population was raised as soldiers to hold the line against the daemonic tide. Ringed by fortress worlds, patrolled by immense battlefleets, and garrisoned by countless regiments, the Gate withstood assault after assault across the millennia, its defenders paying in blood for every hour the passage remained sealed.
The long vigil ended in catastrophe. During the thirteenth and greatest of the Black Crusades, the defenses of the Cadian Gate were finally overwhelmed and Cadia itself was destroyed, its death heralding the tearing open of the galaxy-spanning warp rift. With the fortress world gone and the Gate thrown wide, the barrier that had contained the horrors of the Eye for ten thousand years was broken at last. Its fall marked the beginning of a darker and more desperate age for all of humanity.