The Rangdan were a terrible xenos power encountered during the Great Crusade, remembered chiefly for the catastrophic wars fought to destroy them, the campaigns known as the Rangdan Xenocides. Though the details have been all but expunged from Imperial record, these conflicts are counted among the deadliest the Space Marines ever waged.
What is known suggests a threat of existential scale. The Rangdan menace was said to be so grave that its unchecked advance might have annihilated the nascent Imperium entirely, extinguishing humanity's dominion before it could be secured. To stem the tide, the Emperor committed vast forces to the galactic north, and the fighting exacted a horrific toll, with three Space Marine Legions reputedly brought to the brink of ruin in the effort.
The exact nature of the Rangdan remains a mystery, their true form and purpose lost to deliberate secrecy and the passage of ten thousand years. Whether a single species, an empire, or something stranger still, they were expunged so thoroughly that little beyond the name and the scale of the slaughter endures. The Rangdan Xenocides stand as a grim reminder that the Imperium's founding was purchased with horrors its own histories dared not fully record, victories won at a cost the primarchs themselves mourned.