This sidequest can be obtained after completing the quest Breaching Security, during final conversation with Gabriella. She will give you Olava's token, which can be used to visit a fortune teller and get a reading from her. Note - if you haven't got a token, make sure that you killed Gaius Maro according to additional instructions from Gabriella, which is during his visit in one of major settlements.
Open the world map and travel to Whiterun. Visit the city during the day, finding the Olava the Feeble's House. An old lady should sit in front of it (screen above). Start a conversation and show her the token. A reading differs depending on the current active Dark Brotherhood quest (you can also visit Olava after completing all quests). A second part of conversation is always the same, because Olava will tell you about treasures in ruins of Deepwood Redoubt, where an assassin of old is buried.
The Dark Brotherhood unlocked quest: Miscellaneous: Locate the assassin of old