The Militarum Auxilla is a specialist division of the Astra Militarum that contains most of the Abhuman units of the Imperial Guard. As the nascent Imperium regained contact with the rest of Humanity during the Great Crusade, its servants discovered populations that had developed into true human sub-species. Most of these Abhumans were killed, but some, such as the Ratlings and Ogryns, proved their worth as specialists in the Militarum Auxilla. Indoctrinated in the Imperial Creed, they are taught self-loathing, and watched closely by the Commissars and Priests of the Adeptus Ministorum.
Warhammer 40,000: Rulebook (8th Edition), pg. 71