This quest/content can be acquired by buying in the Bioware Store. It will be automatically unlocked at any point of the game or at the beginning of The Destruction of Lothering quest although its only possible to complete this in Act 1 onward.

To complete this quest during Act 1, you have to go to your Uncle Gamlen's Home in Lowtown  and enter the southern room. During Act 2 and 3 you can find it in the northern room in Hawkes Estate in Hightown. From there you can find inside a chest marked as Special Deliveries a belt named The Antivan Garrote. Upon taking the item, you'll complete this quest.