Take Aveline's advice and go to The Gallows to find Emeric. He will plead with you to help him in his quest to find a mysterious killer. He wants you to search a suspicious house in Hightown at night. Agree and use the map to find the house in question.
You'll know something is going on as soon as you enter the DuPuis Estate, as pretty much every room is infested with Shades and other undesirable creatures.
Search the house for clues as to whether or not DuPuis is in fact the murderer. Clues include vials of blood, a chest full of women's clothing and a letter from Starkhaven.
DuPuis has plenty of excuses when you finally confront him in his room. After you've heard his story, it's up to you whether or not you want to kill him or let him go.