Sisters Oblatia are Sororitas who have taken the penitent's oath, seeking not to atone for some direct misdeed of their own, but through profound spiritual conviction to take on the weight of another's sins, most commonly a blood relative, some infamous ancestor or stain on their family line, or, more rarely, for the past transgression of a friend, a people, or a whole world.
Most Oblatia come to their vows through spiritual revelation or after deep meditation of the nature of their holy vocation, although some few may have the oath in mind as the central cause of becoming Sororitas in the first place. Regardless of reason, only fully ordained Sisters may take the oath, and they must go before a specially convened synod of their superiors who judge the oath's validity and purity of intent, to ensure it owes nothing to hubris or vanity.
The Oblatia must then undergo ritual fasting, mortification, and a symbolic severance from her life and Order before taking up the blade and making her vows before the God-Emperor; turning her back on her Sisters as a requiem mass is sung for her as if she were already dead. Although she seeks ultimate atonement by the manner of her death, the onus on her is to atone through her deeds, most specifically in defeating humanity's great enemies — combating and defeating the heretic, the mutant, the witch and the alien without regard to self, injury or hardship. Through the Oblatias' battles, victories and ultimately the manner of their death, they hope to gain for the object of their sacrifice some measure of forgiveness in the Emperor's eyes.