You can get it by giving Anthony Rohan's seed (usually called "faun's seed") when he's knocked out in his cell, this should trigger the scene.
thx!
how do I collect Rohan's seed tho? I know how to get the flask but how do I get his seed?
It's in the lobby upstairs, where you would normally find Amaïr resting. In order to get it you must first let him milk Rohan during the intro sequence.
thx
