Halo Zone
The Halo Zone is a region of Segmentum Pacificus where the light of the Astronomican doesn't reach. Most people avoid the region, as Imperial Navigators are forced to navigate blind there, making them more likely to get stuck in the Warp or worse, and the time it takes to cross a region of space takes longer, and Astropaths would be beyond the range of communications.
Lord Solar Macharius attempted to continue his crusade to the region, but opposition from his troops and generals prevented him. The Guardians of the Covenant chapter stand guard against it.