Achilus Crusade/Jericho Reach

The Achilus Crusade was a secret Imperial Crusade that began in late M41. It sought to bring the Jericho Reach in the Eastern Fringe of the galaxy back into the dominion of the God-Emperor of Mankind. It had some initial success, but the forces of the Imperium became too spread out, and eventually ran into serious opposition, including a Chaos Anomaly, a Tau Colonization Effort, and a Tyranid Hive Fleet. 

With the final death of the Corpse-God at the eve of the 41st Millennium, and the coming of the Granite Prince in the early 42nd, the Imperium greatly reinforced the Crusade, and evacuated billions of citizens through the Warp Gate, using them to settle reconquered worlds in the Jericho Reach.

At some point, prior to the Fall of Scintilla, all communication with the Achilus Crusade ended. It is thought that Tau forces managed to seize control of the Gate. What became of the Crusade is unknown.