This quest is given to you by Toni one of the Don's men. He can be found sitting on a bench north west of the blacksmith in the merchant quarter of Harbour Town. Talk to him and he will tell you about Don Esteban's amulet that is still in his house, he will also give you a clue to it is location and how to get into the house. You can now do the quest in two different ways you can walk up to Hernandez a Warrior of the Order guarding the door and tell him about the amulet or you can try sneaking in through the window at the back of the house.