Kill Snakeroot and report to Widowmaker. 

Cast Spirit Vision in sawmill, and you will encounter the spirit of a lizard called the Black Widowmaker - a renowned Lone Wolf poisoner. He himself was tricked into consuming poison by a fellow Lone Wolf, Snakeroot. He wants her dead.

You can consume the Black Windowmaker's spirit, or kill the Snakeroot. Ask for his reward if you helped the Black Widowmaker.