Assassin's Creed III Remastered

To visit Oak Island you have to find all 24 Peg Leg Trinkets and finish all parts of the "Peg Leg" side mission explained in the "Bring Down the House" achievement description. This is just finishing the Kidd's Treasure quest. This is the last location of the quest that you must complete. After you are done, the achievement will unlock (uncovering the mystery is just doing the quest, nothing special on your part).