How to Acquire
During the mission, House of Pleasure, Corvo Attano must eliminate the Pendleton twins and locate Emily Kaldwin. The easiest way to find Emily is to equip Dark Vision, once Corvo nears the bathouse. Emily is on the top floor of the Golden Cat, in a room by herself. After reuniting with Corvo she will run to the VIP exit.
However, the VIP exit is locked, and Corvo must acquire the Master Key from Madame Prudence in order to unlock it. It can easily be taken by pickpocketing her, as she is focused on reading a piece of paper and will not notice Corvo entering her office.