Iggwilv, also known as Tasha or Natasha, and the alias Zybilna, was an Oerthian archmage and demonologist. She famously wrote the Demonomicon of Iggwilv, a tome on demonic lore.
Later in life she created her own Domain of Delight in the Feywild, and ascended as one of the plane's powerful archfey.