Found during the Magi Origin (Bound in Blood and Magic) in the room containing the Tevinter statue in the basement of the Circle tower. At the end of the quest, First Enchanter Irving will ask if the Warden took anything of importance from the basement. The Warden needs to have at least one rank of Coercion to keep the staff. If the Warden denies taking anything of importance and fails to persuade Irving, he will reclaim it. If your Warden destroys the staff, the game will treat it as if you haven't taken it.