The Soothsayer was an oracle who once lived in a chamber deep in Ilirea.
The chamber in which the Soothsayer lived in was built over a fissure, the vapors of which were thought to increase the chance of having a vision of future events. The Soothsayer lived there for centuries, even after the elves had left Ilirea. No one knows where the Soothsayer went after she left the chamber. The Hall of the Soothsayer is where Galbatorix tortured Nasuada in Inheritance. It was said by Galbatorix that the Soothsayer was neither elf, nor dwarf, nor human, but something else entirely. This could imply that the Soothsayer was a descendant of the Grey Folk, as Christopher Paolini hinted that readers would encounter such a descendant in Inheritance.
A common belief among fans is that the Soothsayer is none other than Angela the herbalist: though this has yet to be confirmed. Angela once went by a different name, though it is unknown what it was.