This quest is available in Lowtown during the day from Sergeant Joanna, who stands just south of The Hanged Man. She is offering a bounty for four nobles who fled Ferelden after a failed conspiracy attempt involving the Warden-Commander in Amaranthine. Hawke can turn the bounty in as s/he finds the nobles, or all four at once.
Note: This quest is only available if you imported an Awakening save where the Warden defended Vigil's Keep and allowed Amaranthine to be destroyed, or if you used the "No Compromises" pre-built history.
Bug! If, in Awakening, you saved before deciding whether to defend the Keep or the City, then used that save to first defend one and then restart to defend the other, then using that game as your history import will allow you to play both this mission and Secret Rendezvous.