Talk to Isabela at The Hanged Man then head to the Hawke Estate to find her snooping around. Agree to help her then go to The Blooming Rose. Head to the room on the eastern end, the coversation is up to you and ends the same regardless of what you do. Head to Lowtown at night and follow the trail that was left behind. As you pass the Foundry search a rubble for a Qunari Sword then continue the trail. Head to the Docks at night and follow the trail to Castillon's Landing.
Head north and hit the disarming switch and loot the crates for a large amount of coin, then enter the open area to view a scene. Kill the Raiders and loot Velasco for the Office Key and The Pairing Knife . Search the room to the north to find Ambrosia and a chest - complex . Enter the southern room and check the chest for Incriminating Documents . As you leave the area Castillon confronts you. Either kill him or accept the deal that Isabela wants to make with him to finish the quest.