The Knower of Names was an "Ancient Baatorian" who had knowledge of all true names. She was bound, however, to only reveal true names if a price was paid. She claimed that the price was set by the gods and that she had no control over what that price was, though it was usually an enormous amount of some currency.
After seducing her, Mephistopheles used her to expose the treacherous nature of the devil generals who served him. He then bound each of them beneath the ice of Cania and imprisoned the Knower to protect himself from the power of his true name. Centuries later, Drogan Droganson's Pupil sought her out as part of their quest to escape Cania and defeat Mephistopheles, who had incarnated in the Prime Material Plane and was laying siege to Waterdeep. After being freed, the Knower greeted the hero by their own true name as a reward for her freedom, and provided them with the true name of the Reaper in exchange for the items that had guided the hero through the wastes of Cania, a price she found to be odd. She was willing to tell the hero the true names of their companions and many others for hefty sums of gold.