Survivors of a Dark Eldar reaping, these ladies were infants when their community was ravaged.  Their brothers were committed to Scholam which fed recruits directly into the Departmento Munitorium, for induction into the Imperial Guard; occasionally, depending on need and the individual, into the Commissariat or the Storm Troopers of the Hykerion Rangers.
Dissatisfied with their pre-ordained futures as veterinary assistants, they left en-masse to find a future which allowed some pay back to a hostile galaxy.
They managed to find somewhere where the submachinegun was taught as a martial art.  They all managed to achieve mastery of the firearm.  In the years since, some of them have also become proficient in other weapons.

They have managed to find themselves uniforms and elected leaders from amongst their number.
As a formed body, they've presented themselves to the Ordo Xenos as a tool to be used, a weapon to protect humanity, to take the fight to the Xenos.
They have been sent to Devos IV with the Holy Ordos mission.  There may be Eldar.