It's been a while since I played now, but I did figure this out lo those many years ago.  You have to talk to Ratbone about collecting rat tails and through a very specific order of dialogue that I can't presently recall, you can offer to get him five cranium rat tails.  Once you bring them to him he'll tell you a secret about the collector following the nobleman, and then you can confront him.  I think you have to talk to the abishai in the bar somewhere in there too.

But there is no quest to allow you to use the pendant or anything.  That's where it ends.