Report to Investigator Fezzen Brasstacks in Shadowfang Keep.
You look like you're fit enough to take on the mastermind behind this operation. The ledger says he's holed up in Shadowfang Keep, so that's where I'm sending you.
We have a man there already but he's not fit to take down an operation by himself. Head on in; the investigator will meet you there. And bring friends.