She's the old crone that hangs out in the Bannered Mare in Whiterun at night. After completing the Dark Brotherhood quest "Breaching Security", you'll get Olava's Token from Gabriella in the Sanctuary.
Talk to Olava and show her the token to get a free reading from her. She'll reveal a new location called Deepwood Redoubt and give you the task to locate the assassin of old.