As Samuel docks the boat at the beginning of mission three, he will talk about a man named Slackjaw. Make your to Granny Rag’s Apartment, and opposite it you will notice a thug beckoning you. Talk to him and then enter the Dunwall Distillery and find a man named Slackjaw. Speak to him and he will ask of you a mission, the first of three. Two of those missions have to be completed during mission three, whilst the last one is completed during mission seven. All three of Slackjaw’s missions are optional, so be sure not to miss them. Refer to the spoiler for a brief walkthrough of each mission.
Mission one - you will have to locate one of Slackjaw’s mens final words scrawled on a piece of paper. Make your way to the building marked on your screen, enter and go up the stairs. Kill the two guards near the table and retrieve the note. Return to Slackjaw to complete the mission. **This is to be completed during Mission Three of the main story**
Mission two - Slackjaw will ask you to uncover a safes code for him. Make your way to the Golden Cat and torture the Art Dealer until he gives you the code (this is also the non-lethal way of dealing with the target, so simply stab him if you’re going for high chaos). With the code, make your way back to the Distillery and speak to Slackjaw to end the mission. **This is to be completed during Mission Three of the main story**
Mission three - **NOTE -Load/utilize the save file you made BEFORE beginning Granny Rag’s Final mission in main mission seven** - Towards the end of mission seven you will find Slackjaw held captive by Granny Rag’s. Refuse to help Granny and then speak to Slackjaw. You will have to pick up Granny Rag’s Cameo and then place it in the furnace upstairs. Do so, and then kill her human body. Retrieve the fallen key, unlock Slackjaw, and the trophy will unlock.