Prime Suspect

Head to Hawke Estate and read the letter from the Templars. If you worked for Dougal in the previous chapter you can now either pay him off or confront him in a random fight later on. Either way, travel to the Gallows and talk to the letter writer, Emeric.

Emeric will instruct you to find Gascard DuPuis at his estate in Hightown. Head there at night and you'll be ambushed by Shades and Abominations; use nature based magic to deal with them quickly. Seems DuPuis is a blood mage and has taken a woman hostage to save her from the real killer. You can choose to hear him out or attack him. The latter isn't needed, but can be used to gain extra XP. Either way Gascard will still make his leave from the estate.

Return to the Gallows and talk to Moria about a fake letter that arrived for Emeric. Head to the Blind Alley to fight more Shades and even a Desire Demon. This demon is able to heal, so be sure to take her out first. When Moria arrives after the battle you can choose to arrest Gascard, or let him go free so that he can take part in a future quest.