This is an optional quest in the Overseer Campbell quest area. After you have dealt wit Granny Rags Gentlemen Callers she will give you a rune. She has another mission which I will write a guide about later today. This guide explains how to deal with the Gentleman Callers Non-Lethally and without being detected. This is actually fairly difficult since you can’t take out all 3 by choking:
There is a video at the end of this guide with a visual version of this guide.
You find this quest by going down the right street after you leave the dock area. Granny Rags is dropping something from her balcony when you first walk down the street:
And go to the archway to hide behind. Be sure to be sneaking, otherwise the Gentlemen will notice you instantly. You have to wait out their conversation, otherwise you will be noticed as soon as you take out the first gentlemen. After they have stopped talking approach the closest gentlemen from behind but to the right to avoid being detected by the gentlemen standing near him. As you choke him pull him backwards and hide him near the archway.
The other two gentlemen can’t be taken out with choking (at least not on the hardest difficulty). Use your sleep bolts and the crossbow to take them out. I don’t think there is a need to hide the bodies. You can replenish the two sleep bolts inside Granny Rags house on the bottom floor. There is a room full of loot to the right of the main entrance (as you enter the house through the main entrance).