The quest may be started any time after Hawke has arrived in Kirkwall and completed the quest The Destruction of Lothering. In Act 1, the quest is started in Gamlen's House by interacting with the griffon statue in the storage room to the east of the main room. In Act 2, Act 3, and after the post-end game save, the quest is started in the Hawke Estate by interacting with the griffon statue in the upper-level room in the northwest corner of the house.
Interact with the griffon statue, as mentioned above. You will be asked to select a party. If you are playing Legacy in the post end-game save, you will only be able to select party members who survived the events of The Last Straw. You will be able to select Carver Hawke or Bethany Hawke if either remains alive.
A cutscene is triggered showing a new part of Cassandra interrogating Varric. Cassandra tells of a report of a chasm in the Vimmark Mountains and states that when the Chantry went to investigate, they were turned away by Grey Wardens. Varric agrees to tell the story.
The action cuts back to the Vimmark Wasteland, where Varric explains that this is the Carta hideout and that the Carta have been targeting Hawke. Varric will be present to give this introduction even if he is not in the active party.
Hawke gains access to the Vimmark Wasteland and the quest Legacy is triggered.