Use your Witcher Senses to re-trace the victims steps across the street and to a small wooden structure around the corner. Enter the building.
Inside you will find three dwarfs ransacking the house. It will turn out that they are Cleaver’s allies and that they have murdered "Derrick", as they call the victim. What is more, they have orders from Cleaver to kill anyone who finds the den.
You will now have two choices.
