During Book One: Reborn, you must make a promise to the dying prisoner, which can only be done in Chapter 2: Friar's Keep.
This achievement can only be unlocked during Chapter 3: Marcuria, where Kian needs to do the following goals:
*** Spoiler - click to reveal ***The Mole: Seek out ‘the Mole’ near Shady Quay.
Traitor: Identify the Azadi collaborator in the Oldtown market.
Optional: You made a promise to Arn Stont. Seek out his widow in the Bones.
The interesting thing about this is that your Book One: Reborn's choices determine if you receive the optional goal. If you made the promise to the dying prisoner during Chapter 2: Friar's Keep, you'll get the optional goal in Book Two: Rebels' Chapter 3: Marcuria. If you want to make sure that you've made the right choices for the dying prisoner.
At the start of Chapter 3: Marcuria, you'll have three goals to complete, but it's highly recommended that you focus on the optional goal first. Head to The Magic Market and a cutscene will automatically trigger as you get closer to the center. During the conversation with the boy thief, Bip, select the following:
This will allow you to really start goals #2 and #3 at the same time, but the first thing you should do is to keep the promise of delivering Arn Stont's message to his wife Gerdi. She's found in The Bones which is on the western side of Marcuria. Follow Bip and he will tell you to meet him on the other side. You'll see two Azadi soldiers standing guard by the Bone's open gate passage. As you walk closer, the screen will turn gray. You'll be safe due to the veil magic potion and will pass through. You're now in The Bones, head up the west path and keep going until you find a Woman sitting on some stairs on the right side. To make things easier, there is a sign that reads "*Stont Family* Carpentry Construction". Walk up to this woman and interact with her and the achievement will unlock.
Credit goes to www.playstationtrophies.org