The quest is initiated by talking to Roger the Fence, a Shadow Thief merchant in the sewers. He will mention having troubles with "the beasties". Offering to help will lead to him asking Gorion's Ward to kill a sea troll.
To find the troll, simply head south along the tunnel from Roger's location, then on the crossroad head west. The troll should be somewhere around the corner. To effectively kill the troll, the final blow should (as usual) deal acid or fire damage.