On the seventh day, start the main quest, but don´t finish it. Before you do it, talk with Mark Immortel and tell him you need an executor´s costume (you know, big bird mask and blackish cloak). He will tell you that there is one in the "pit". Said pit is right in front of the cementery, is a small square with a bunch of corpses. Outside is a single corpse, click on it and you will get the costume (because of the tall grass it can be hard to see).
Now, go with the Inquisitor and use your manly charms to convince her to spare Peter Stamatin. After that, go and talk with Peter, and he will give you the executor´s mask.
Finally, equip both the costume and the mask, and you will unlock the achievement. This things have zero protection, so you better not use them again.