After your dialog with Tallis and your subsequent escape, you have the choice of heading straight up into the vault room or making your way through the dungeons and then up into the vault room. The latter route grants more XP.
After a short cutscene in the library with Prosper and a battle, you can head down into the vault room. Clicking on the book on the floor triggers a cutscene with Aveline where she will read the contents of the book and give you the next part of the quest.
After you are finished robbing Duke Prosper, head back down to the Dungeons via the door in the vault. This door will take you near where you need to be. If you are a rogue or if you haven't had Tallis or one of the other rogues in the party (Varric, Isabela, or Sebastian) unlock all the cells now.
The second cell on the left is where you want to be. There will be a gold arrow pointing to the floor; click on it and it will start another short cutscene where Aveline finds a black vial and smashes it.This unleashes a revenant and skeletons. Killing them triggers another cutscene with Aveline that ends the quest, leaving a lot of questions unanswered for everyone involved.
The number of skeletons spawned changes depending on how many prisoners you released (4, 5, or 6).
- 230 XP (for the fight) and 200 XP (quest completion)
- random loot
- Signet of the Red
100% of basic attack damage vs. enemies that attack in melee
The mysterious vial Aveline finds and picks up in the cell pays humorous tribute to the unmarked Black Vials quest from Dragon Age: Origins. Most companions will use the opportunity to lean on the fourth wall by cracking jokes about certain aspects of said quest, like the incantations' bad verse or the question why something as obviously dangerous as the Black Vials seems to be lying around everywhere without proper protection.