|The Rangdan Xenocides|
|Date||ca. late 860s.M30 - ca. 930s.M30|
|Theatre||The Halo Stars|
|Rangdan Cerabvores||Imperium of Man|
|Commanders and Leaders|
|Unknown||Emperor of Mankind, Leman Russ, Lion El'Jonson|
|Rangdan Cerabvores, Rangdan Osseovores, Slaugth Murder-Minds||Dark Angels Legion, Space Wolves Legion, War Hounds Legion, Death Guard Legion, Alpha Legion, II Legion, XI Legion, Legio Gryphonicus, Legio Vulturum, Legio Kydianos, dozens of other Legio Titanicus Legions, Knight House Malinax, Knight House Orhlacc, Xanite Mechanicum Taghmata Forces, Centurio Ordinatus, Ordo Reductor|
|Unknown, probably total||Massive losses among Imperial expeditionary forces, entire Titan Legions and fleets destroyed, possibly two Legiones Astartes destroyed|
|Elimination of the Rangdan threat to the northern Imperium; continuation of the Great Crusade; depletion of the Dark Angels and Space Wolves Legions; Ultramarines become the largest Space Marine Legion|
The Rangdan Xenocides were collectively the largest and nastiest fight the Imperium of Mankind faced during the Great Crusade. This was a conflict so horrendous that it swallowed entire expeditionary fleets and Titan legions, nearly crippled the Death Guard, the Dark Angels, and the Space Wolves, and saw dozens of worlds laid waste. Most interestingly, it may or may not also be the reason that the II and XI Legiones Astartes were wiped from all Imperial records.
The Xenocides began sometime in the late 860s.M31, around the time that the Imperium was entering the Eastern Fringe of the galaxy. As they were expanding toward the Halo Stars, they drew the attention of the Rangdan Cerabvores, a mysterious and hostile xenos species who decided to tell these upstart humans to get the fuck off their lawn. Unfortunately for the Imperium, they had the will and the technology to make it happen, and would in fact prove to be the single greatest threat to its existence up until Horus's little temper tantrum.
The Rangdan attacked from the galactic east and north, overrunning entire worlds and putting the Imperium onto the back foot with shocking speed. Fleet after fleet was lost with all hands, entire Titan legions were obliterated, and dozens of planets were destroyed as the Cerabvores continued their relentless assault. The Imperium wound up throwing no less than seven Legiones Astartes at the Cerabvores: the Dark Angels, Space Wolves, War Hounds, Death Guard, the II and XI Legions, and eventually the Alpha Legion. Even this wasn't enough, and the destruction continued unabated for decades. The Space Marines committed to the conflict sustained heavy casualties. The Dark Angels especially got their shit kicked in; by the end of the Xenocides, they had lost their status as the largest Legion to Papa Smurf's blueberries. The Death Guard had to conduct emergency intakes of recruits just to keep their numbers up, and the II and XI Legions apparently ceased to exist during or shortly after the conflict, hinting that they may have been corrupted or otherwise compromised by the Rangdan and needed to be purged. The Lion received his baptism of fire in the Xenocides, as he had been rediscovered in the midst of the campaign and was almost immediately put in charge of the entire war effort. Despite his sheer Tactical Genius, the bloodshed continued, culminating in the Third Rangdan Xenocide around 890.M30, which successfully preserved the northern Imperium at the cost of 50,000 Space Marines.
Eventually, things got so bad that the Emperor concluded he was going to have to do something incredibly dangerous: open up the Labyrinth of Night and unleash the motherfucking Void Dragon in an effort to break the Rangdan once and for all. Whatever happened, it totally worked and didn't cause a galactic calamity that we know of. After that, the only thing that was left to be done was for the surviving Rangdan to be slowly, bitterly, painstakingly purged from the galaxy in a series of "bio-pogroms", a task which fell to the Space Wolves and the Dark Angels, as they were the two Legions the Emperor could most rely on to perform such an ugly and brutal job. This last phase of the conflict took a decade, as the two legions scoured the sector clean of the xenos menace. This earned the Space Wolves their nickname of the "Emperor's Executioners".
After the end of the Xenocides, the Dark Angels and the Space Wolves had both been greatly reduced in number, meaning that the progress of the Crusade now rested on the Luna Wolves, Ultramarines, and the other Legions that hadn't been sucked into the meat grinder. The I and VI Legions also became objects of suspicion and fear among ordinary Imperial folk, thanks to the sheer ugliness of the bio-pogroms they'd undertaken. The Ultramarines officially became the largest legion, a status they would maintain throughout the remainder of the Crusade and into the Horus Heresy.
The conflict with the Rangdan also spurred the creation of newer and more terrifying weapons of war for the Imperium, most notably the dreaded Psi-Titans of the Ordo Sinister, which were directly inspired by the Rangdan's own superheavy and Titan analogues, the Macrobeests and Osseiovores.
Interestingly, the Xenocides were also the first time "Alpharius" was heard of, even though the primarch of the XX Legion had yet to be recovered at that point. An Alpha Legionnaire using that name gained an audience with the Lion at some point, and offered to secretly take over the war so that the Dark Angels could withdraw and rebuild their numbers. The end goal, the Legionnaire explained, was to keep the Dark Angels from losing their status as the biggest legion and therefore improve the Lion's chances of someday becoming Warmaster of the Imperial armed forces, a position which both the Lion and "Alpharius" believed would be inevitable. The Lion turned him down because he was determined to finish what he'd started, which went well for his Legion.
The mention of Slaugth Murder-Minds as part of the Rangdan forces also ties the Xenocides into the 41st Millennium via the xenos species of the same name mentioned in the Dark Heresy RPG. Not surprising, though, since the fluff for the Dark Heresy system and the first several Horus Heresy campaign books were both written by Alan Bligh, Emperor rest him.