Story related and cannot be missed, you will get this after completing Chapter 3: Dirty Secrets.
WARNING: There is a glitch in this mission, sometimes a rope you need to climb at the end will not appear. This can be countered by buying rope arrows before the mission, these can be used to bypass the glitch. Make sure you have rope arrows and don't lose an hour of progress like I did!
Here is a walkthrough on Master difficulty if you need it, this is not my video and all credit goes to the Youtube channel Front Towards Gamer: