Top of the City Quest/Spoiler

Go to the Former Trade Quarter and search for Iriana.

Talk to her and say "Mission"

She will tell you that a burglar has stolen her income and he started to run away on the roofs.

Go inside her house, and head to the roofs. Then you will quickly see a white assassin appearing, turning around, and vanishing.

Keep following him, and try to find the stolen goods. Meanwhile you need to kill all kinds of outlaws.

Watch out, you may get trapped by a thief. And then you need to go back to Iriana. The same counts for if you lost track of the white assassin.