It was one of the earliest worlds conquered by the Imperium in the Great Crusade by the First Legion of Astartes. There, the 3rd Chapter of the Legion, formed mainly from the Hosts of Stone and Iron, assaulted a world whose mountains and crust had been hollowed out to form a fortress for a swarm-like creature of protoplasmic and hyper-acidic slime. The creature's nucleus had spread throughout space and infested a dozen worlds. The First Legion's 19th Expeditionary Fleet engaged the vast sentient planet-killer in a battle that was later deemed classified by Legion Master Hector Thrane. The details of the battle and the creature encountered have been kept secret to preserve the sanity of those who may learn of it.