Tanova was a spell-casting vampire who was part of Bodhi's vampire guild. Unlike other vampires, she could cast spells, making her a larger threat. According to Centeol, Jon Irenicus once met a human named Tanova in Baldur's Gate, though it is unknown if they are the same person.
If you sided with the Shadow Thieves in Baldur's Gate II Chapter 3, she will be guarding the corridor to the torture chamber, and, being able to cast spells such as Confusion and Hold Person, she can be very lethal to an unprepared party. Regardless of whether you have already killed her or not, if you stay long enough in Athkatla before leaving to save Imoen, you will find a man named Sansuki running from a group of vampires, Tanova being one of them. If you have already sided with the Shadow Thieves, the group will attack you. Killing her here merely transforms her into a mist; she will still be fought in Bodhi's abode.