I used to make these sorts of posts a great deal more, but I stopped because I didn't wish to provide spoilers for my online group - who reads this blog regularly. Since this villain is now gone, I can discuss her in some detail.
During the last generation to know the Sword Brothers before the siege of Nightwick Abbey, the Baroness came to the Dark Country from the county of Averois in the Realm. Her name is lost to, or perhaps deliberately hidden from, time. In the chronicles of the Sword Brothers she is referred to only as the Baroness, and those few sources from that era written by others that still survive she is called the Witch of Averois. She was the sole heir to a barony in the forest choked province of her birth. During her youth she proved to be a difficult child and one with a gift for manipulating others. As she grew older her manipulative nature combined with her burgeoning womanhood into disturbing turpitiudes. Some legends tell of dalliances with both men and demons, but surely this could not have happened before she came to the abbey.
Her father sent her to the Dark Country to stay in Nightwick Abbey so that she would learn holiness at the feet of the Sword Brothers. In hindsight this move was extremely counterproductive, but one must remember that at that time the truth of the Sword Brothers was not yet known - their depredations of the countryside considered evidence of their unflappable devotion to the law instead of the influence of the Pit. It is said that he sent her there out of fear for her mortal soul, but there are dark whispers that even then Nightwick's taint was such that it could call out to the evil of heart. Perhaps it called her and she forced her father to allow her to leave.
From almost the very beginning her acts were blasphemous, and many think that it was her presence that first revealed the true evils in the abbey. She dressed herself as the Lady and demanded to be addressed as such. She would further modify the Lady's traditional garment to expose aspects of her womanly figure in deliberate mockery of the Lady's chastity. A segment of the Sword Brothers indulged her in this lustful play acting, often in public for all to see. She would select pagan captives to be brought back to the abbey, and many at the time thought these victims were to provide her with sexual favors. However, when some escaped the horrors of the dungeons beneath they recounted how she delighted in the torture of her chosen victims and also had her devoted Sword Brothers visit pain upon her, forcing them to drink her blood as she drank the blood of her victims.
Some say it was she who first invited magicians to the abbey to begin construction of the Sunless Garden and the twisted army they failed to perfect in time for the siege. It is certainly true that she aided their efforts with her perverse knowledge of human anatomy - gained over her years of torturing captives - and that many of the forms of undead created within the abbey are her handiwork. Notably those referred to as the Caged Ones and the Broken Dead were likely created from her necromantic experiments.
When the forces of the West finally stormed the abbey, some sources say, the Sword Brothers had been split into two factions. The most powerful and largest worshiped a demon that, in the year 1393, was determined to be none of than Armadeus, the Emperor of Lies. However, a smaller cult dedicated to "the Lady" (that is, the Witch of Averois), had splintered off and refused to acknowledge any power higher than her. When the men of the West came to the torture chambers under the abbey, they found these two cults were already slaying each other - despite the fact that the abbey was under siege from outside forces! The slaying of her mortal form is not recorded, though it clearly must have transpired given the events that followed.
In the years after the fall, rumors circulated of robed figures entering the ruins to pay homage to "the Lady." Those about the village of Nightwick believe she was fed on the blood of dogs, pigs, and other animals taken to the abbey in order to give her unnatural life. The Howling Kommandos first found evidence of her existence on the first level of the dungeon, where depictions of the Lady seemed to represent an oddly specific person rather than an icon of the church. Kozel, a devotee of Saint Santa Claus, began to research this figure and discovered the title of the Witch of Averois.
Soon after the Kommandos began encountering cultists who clad themselves in black robes covered in stylized blood drops. The worst of these were the berserkers, who added human skin leather to their uniform and, according to a cultist they interrogated, drank blood from "the Lady's" teats. Eventually they made their way onto what they called "the Baroness" level, which seemed to be comprised mostly of her old torture chambers. After avoiding her for many sessions, the Kommandos were eventually able to build up a posse of enough men to dare to venture into her lair. They slew her and even thought to cover her blood-drenched remains with holy water to keep her from rising again. As of their last delve it seems to have been effective. Her cult is dispersed and her evil is no more.
However, it is known that there are many "instances" of the abbey and that the realities within are particular to the men and women who seek to wrest fortune from its darkened halls. Perhaps, in some other abbey, she still lives.