They are a nomadic alien race comprising many genetically engineered forms created from harvested bio-mass. They are known as the "Great Devourer" and also the "Shadow In The Warp" (after the effect of their hive mind on FTL travel and communications) and pose a severe threat to the Imperium. They seek to consume all in their path, draining all planets of every resource and converting all living and some non-living material into Tyranid biomass.