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.