Unlocked in chapter one, just after the tutorial. The objective is to steal jewelled mask from Lyegrove.
1. Enter the jeweler from the back door.
2. You should be in a basement with a guy working at a desk.
3. Locate and crack the safe. (The instructions are beneath, but if you'd like to find it and combination yourself i will leave it as a spoiler).
*** Spoiler - click to reveal ***Safe is located behind the painting, right to where you entered, just remeber to extinguish candles next to it, combination can be found in a letter lying on the desk where the owner is working, it is: 7-3-9
4. After picking up the mask achievement should pop up.