The Sisters of Battle are the military wing of the Adeptus Sororitas, itself a branch of the Ecclesiarchy. The Sororitas are an all-female religious organization that further the interests of the Imperial state religion. They were originally an obscure religious sect called the Daughters of the Emperor, before being recruited by Goge Vandire to be his personal bodyguards. After the reformation of the Ecclesiarchy under Sebastian Thor, the Ecclesiarchy was banned from having men under arms; the Sororitas were kept around as, being all-female, were exempt from this prohibition.
As an army, the Sororitas focus primarily on purging heretics, and are often called upon to support various military campaigns or, more commonly, to serve as the frontline troops of the Inquisition's Ordo Hereticus. They are between Space Marines and Imperial Guardsmen in terms of effectiveness, using their own versions of power armor and bolters, and favoring flamer and melta weapons. They have a unique advantage in combat, in that their faith in the Emperor grants them minor miracles.
TTS Canon Edit
Upon the Emperor's declaration that the Ecclesiarchy was to be disbanded and that he was not to be worshiped as a God, the Sisters who were not already exiled into the Warp fell under a collective depression and started drinking heavily. The Emperor ordered them to return to their duties, and promised to send them pinup calendars featuring his glorious golden pecs as incentive.