How to unlock: Talk to Dorian in Skyhold.
Quest giver: Dorian (M12,4)
Talk to Dorian (M12,4). After the conversation, you can talk to Mother Giselle (M12,4a) in the main hall. She will tell you about a letter for Dorian and ask you to take him to Redcliffe tavern (Hinterlands) (M3,2b) for a meeting. Take him as your companion and go to the tavern.
A cut-scene will start, during which the best way is to support Dorian all the time to gain his friendship. When the conversation is over, there's one more dialogue in the Skyhold and you will be complete the quest.