Into that Vast Expanse
Eye of Terror
The Eye of Terror is a massive Warp rift, where the Warp co-exists with real space, the largest and most well-known in the galaxy. It is located at the edge of the galaxy, to the north and west of Terra, in the Segmentum Obscurus.
The Warp and real space are indistinct and intermixed within the Eye, and the whole region is a maelstrom of psychic energy, impossible to navigate through. Imperial ships avoid the area around the Eye for thousands of light-years. Ships traveling too close to the Eye can be thrown far off-course – they can also be caught in temporal whirlpools which carry them backwards or forwards in time, or which trap them in limbo forever.
The Imperial world of Cadia lies nearest to the best-known stable gateway into the Eye; consequently, it is the most fortified and militarized world in the Imperium, the first line of defense against Chaos incursions into real space. It would seem that ships exit through this gateway far more often than they enter it.