"There exist out there in the unbroached void of night and in the deeper darkness beyond, things so terrible and vile that they would devour us and destroy everything we have made. And why? Because they fear us—and rightly so, for we are humanity and the stars are ours."
The Halo Zone is the name of the region beyond the light of the Astronomican that lies in the Segmentum Pacificus but beyond the borders of known human space. This region is composed of the ancient stellar bodies known as the Halo Stars. This thin band of old stars traces the circumference of the Milky Way Galaxy and is home to ancient worlds where men have never known the Emperor of Mankind.
Their ancestors left Old Earth over thirty thousand standard years ago at the very dawn of human interstellar history. So dimly does the Emperor's light shine at the fringes of this region of space that even Navigators can sense only darkness. Due to the resulting difficulties in navigation in this region, the time needed to cross it is greatly increased from a similar volume of explored space, while also increasing the time differential experienced between travel in realspace and travel in the Immaterium.
At the end of the Macharian Crusade led by the famous Lord Commander Solar Macharius in 399.M41, after seven standard years of campaigning and having brought nearly a thousand worlds under Imperial control, his expeditionary fleet reached the edge of known space. When the Lord Commander attempted to push his Crusade onwards into the Halo Zone, it was here that his generals wavered. They pleaded for him to reconsider. His men, tired and ageing, hesitated. The halo of the galaxy was dark and forbidding. Navigation was slow without the guiding beacon of the Astronomican.
The Astropaths were virtually beyond the range of psychic communication. There was a sense of growing unrest amongst the armies and fleets. Macharius knew that the end had come. His armies had simply run out of energy at the moment of his greatest challenge. To make matters worse, some of the exploratory teams had failed to return from their missions, whilst others reported mysterious phenomena. The troops whispered that the Old Stars were haunted, that the worlds which orbited them were inhabited by ghosts, and that the halo was not a place for living men.
Eventually, the Lord Commander gave in, and with a heavy heart he ordered the fleets to turn back into the Imperium. Though his soldiers cheered for their hero and his generals sighed with relief, legend tells that Macharius was a broken man because of his inability to continue his conquests and he eventually died of despair on the return journey.
- Codex: Imperial Guard (2nd Edition), pg. 32