To find the [Aged Envelope], which begins this quest, you must first kill Lieutenant Benedict in Tiragarde Keep on the second floor and retrieve [Benedict's Key] from his corpse. This key in turn will open a chest on the third floor of Tiragarde keep, which contains the item.
When you use the Aged Envelope and accept the quest, you will lose the Aged Envelope and receive [Admiral Proudmoore's Orders] in its place, which is required to complete this quest.