You get this quest if you do the BONUS objective for the quest “Breaching Security”. Gabrielle will have given you an “Olava’s Token”. Head over to Whiterun and see her (She’s an old lady that likes to sit in front of her house). Show her the token and she’ll agree to see your future. She’ll give you a vague view of things to come, but will also tell of an “Assassin of Old” who has met his end in a ruin and has things for you. Once she’s told you all of this you’ll get another miscellaneous quest: “Locate the assassin of old”, which is in the next section.