The Last Mistress

Head of the Buried Hands, an undead thieves' guild in Lekar.

The Last Mistress was first mentioned in Van Richten's Guide to the Ancient Dead, as the foe of a group of knights that had tracked down her guild. Based on references there and in Gaz2, the following writeup was assembled as part of the "Bait This Hook!" project on the FoS board:

The Last Mistress was a notorious burglar born in the reign of Falcon the Great who was not well inclined to Vlad Drakov's government. She staged several increasingly outlandish politically pointed stunts against the new regime, and when the authorities closed in on her, she committed suicide in such a way as to facilitate her reanimation decades later.

By the time she returned, she had assembled a cult following, a guild of thieves that eventually drew the attention of Drakov's forces. The guild was tracked to their underground lair by a group of Raptor Knights, who collapsed the entrance and thereby buried the guild alive. The Last Mistress worked her revenge on the knights, only to run afoul of Rudolph van Richten, who imprisoned her using chains forged from her own hoard.

As of her writeup in Ravenloft Gazetteer Volume II, she's been freed again. The old guild have returned as undead, and they make their base in Lekar.