This can be unlocked in Chapter 2 of Nightfall. It is also missable.
Proceed through the first area where you have to unlock the drawbridge via two levers. Once you advance through the door, you will be tasked with following the messenger while staying out of sight. For the achievement, what you need to do, is run up to him and instantly kill him to unlock the achievement. Then reload and play through as normal for the remaining achievements.