From publisher blurb:
Black Annis. In mythology, this name references everything from the mother of hags to one of the first supernatural monsters to walk the physical plane. But whatever truth may be to those legends, the present day reality is much simpler: There is more than one Black Annis.
When any creature with untapped spellcasting potential is reviled and cursed by the community, her magics can react by transforming her into the object of their fears. This process usually leaves the Annis looking like a wizened old crone, but with the strength and speed of a monstrous predator. This transformation pays no mind to the previous alignment of its victim – much to the surprise of angry mobs in pursuit when it occurs.
Sadly, it is often frightened commoners never aware of their spellcasting abilities who suffer the fate of a Black Annis. If the curse is removed, it is possible to rehabilitate a Black Annis back into society, but doing so is often no easy task since she will resist at every turn.