Summary
Running from 832 to 845.M41 the Black Sun Crusade takes its name from the Black Sun of Medrengard, fortress world of Perturabo. It retrospect it seems clear that the incursion in force was the intended first blow of a new Black Crusade. Had Perturabo achieved a full warp transition of Medrengard and established himself in Aliris Sector it would have been possible for Abbaddon's forces to avoid the defenses around the Eye and drive directly for Holy Terra.
Aliris Sector
Is well known as the location of the Hadronian Wall, a multi-system line of defenses strung along the rimward front of the Sector and anchored by the Fortress worlds of Sarkomand and Malakand. The Hadron System is at the exact center of the line and consists of the twin Fortress planets of Hadron's Sword and Hadron's Shield. Sentinel stations established far forward of the line are known as Hadron's Eyes keep a constant watch for Enemy movement from the Eye. This single minded focus on the rimward front nearly cost both the sector and the Imperium dear.
Imperial Forces Involved
Chaos Forces Involved
Others
The War In the Reach
The Reach Campaign
Prelude
The Podromos Reach is a haunted section of space between the inner and outer spiral arms, dotted with dead worlds and dangerous secrets, and only sketchily controlled by the Alirian Sector Lords all of which goes a long way towards explaining why the periodic appearances of a 'Black Sun' radiating warp effects shattering human minds and civilizations failed to attract anything like the attention such a phenomenon deserved. At least fourteen sightings with effects ranging from scattered chaos cult break outs to full scale civil wars have been documented by the Arilian Inquisitorial conclave - very much after the fact. At the time however the stories were dismissed by the Administratum on Liriopis as 'Reach tales'. Local authorities, military and civil, were less dismissive but as the alien Sun had invariably vanished well before the first response ships arrived they had no evidence but the hysterical and possibly delusional ravings of the victims and no way to judge the nature of the threat nor did analysis reveal any pattern to the appearances.