Way to unlock: Find a note in the stable - Blackwall is not there at that time. Available after completing the main quest "Here Lies the Abyss".
The quest-giver: - (M12,18)
After completing the "Here Lies the Abyss" quest, when you go to the Skyhold and then to the stable instead of Blackwall you should find a hidden letter there - unless first you must watch all cutscenes related to him - in that case it is possible that you will need to leave the location so that Blackwall "can disappear".
After you read the letter a messenger will show - talk to him and go to the Val Royeaux. There, on the main court a large group of people will gather around the gallows - walk towards them After cutscene go to prison in southern part beyond the gate, near stairs. In prison, talk to Blackwall and then with Cullen. Now you can go back to Skyhold and decide what to do with Blackwall.
Two operations are available: "Thom Rainier's fate" and "Rainier's Release: An Alternative". No matter which you choose, both of them will end with getting Blackwall to Skyhold for your judgment. No matter of the judgment you will unlock "The Consequences of Blackwall lies" operation.
There are three options of judgment available:
- Atonement - support of companions
- Giving to guards - small support of companions
- Keep Blackwall as servant - no support of companions
Rewards for completing the quest:
- Experience (lots)
- 400 influence points
- 3 power points