Employer: Basso [Stonemarket - Basso's Office]
Availability: You get the opportunity to start this mission, after you have completed the sixth chapter of the game. You need to meet up with Basso again, at the Crippled Burrick tavern.
Your destination has been presented in the above screenshot - it is located in the central part of the Old Quarter, in [Stonemarket - Muddler's Row], to the North of the watchtower. To learn more about the suggested way to reach this area, see the page entitled Reach the cathedral in Old Quarter. I recommend that, before you start this mission, you make sure that Garret has the Wrench on him.
Stop to the South-West of the marker and find the venting shaft shown in the above screenshot. Use the wrench to access the shaft and walk through to reach the witch's place. The most important thing that you can find here is the ceremonial blade (Ceremonial Blade - 8+50G), but you can also find here a Poppy, a book (Stonemarket - Candlewax Ritual Book) and the diary (Stonemarket - Wax Witch's Journal). This job ends at the moment at which you pick up the blade.