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. It is therefore a site of frequent raids by the forces of Chaos seeking to break through the Imperial defenses in the region so that they can invade the larger Imperium.
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 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 world of Cadia during the 13th Black Crusade in which minor Chaos victories resulted in the capture of a small Imperial settlement.
The Cadian Gate is not the only stable exit from the Eye, the Arx Gap is another way to get out of 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. It bypasses the powerful defenses of the Cadian Gate, as such making it a more attractive location for the forces of Chaos to stage their assaults.
- Codex: Eye of Terror (3rd Edition)