- Kill four guards in one kill streak
- Use 10 Blowpipe darts on guards
Make your way to the exclamation point in Nassau and talk to Charles Vane to start this mission. The good news is that there is no detection limitations, in fact you will need to get in open conflict to get the four kill streak for the optional objective. To complete the kill streak objective, find a large group of standard guards and throw down a smoke bomb to get at least 3 and then combo the fourth in at the end (you can create kill streaks by pressing the attack button and holding in the direction of your next target).
Before you grab the last powder keg, make sure that you have used 10 darts on enemies. If you don't have a lot on you, you can get more through crafting or by looting defeated guards.