I try to do the quest Mysterious killings in Vivec. I have heard of this in "latest rumours", had speaken to Elam Andas a few times (quest upgraded twice), searched all Foreign, Hlaalu, Redoran and Arena quarters and nothing happens.
I have not found anyone who may provide any useful info (UESPWiki says Marcel Maurard is a good NPC to ask about but he has nothing interesting to say - maybe I need to do the quest associated with him?). Of course, I visited most of Underworks (especially under Foreign Quarter), killed a lot of rats, cleared a few Daedric places of worship etc. but I have met no murderer.