The Rangdan were an incredibly vicious and technologically advanced species that assaulted the Imperial forces during three campaigns remembered as the Rangdan Xenocides, starting in the decade of the 860s.M30 and ending in the early tenth century of the 30th Millennium.
It was the bloodiest conflict the Imperium would fight in its earliest days before the start of the Horus Heresy.
While the Rangdan were ultimately defeated, it required the direct intervention of the Emperor Himself and at least three Space Marine Legions, over a dozen Legio Titanicus Titan Legions, several Imperial Knight houses, the military forces of the ancient Mechanicum and hundreds of thousands of troops of the Imperialis Auxilia.
The conflict would do lasting damage to several of the Space Marine Legions. The Dark Angels took the brunt of the casualties and the war against the Rangdan would be the reason that the numbers of the I Legion dwindled to the point that the Ultramarines would eventually overtake them as the largest of the Legiones Astartes.
Very little about this species is known, since the records surrounding the conflict were sealed long ago.
Reports say that the Rangdan attacked with two forms of warriors, "Rangdan Cerabvores" and "Rangdan Osseivores."
The Rangdan were feared by the military forces of the Imperium for their skill at mimicry.
Rangdan starships were characterised by a somewhat organic appearance, possessing spines and flails which dragged behind their vessels like metallic tentacles, which gave them the appearance of Terran jellyfish.
- The Horus Heresy - Book One: Betrayal (Forge World Series) by Alan Bligh, pg. 99
- The Horus Heresy - Book Seven: Inferno (Forge World Series) by Alan Bligh, pg. 81
- Scions of the Emperor (Anthology), "First Legion" by Chris Wraight