Gamlen's enquiry of Leandra's well being stirs concerns about her safety. Fearing the worst, you can do one of two things:
Follow the trail of blood to the Hidden Trapdoor on the second level. You'll have the standard fare of Shades, Rage Demons, Corpses, and other undead to deal with. There's a lot to pillage, including Orichalcum, so be thorough in your search for loot.
When you arrive to face the captor, interesting things unfold if DuPuis is in the party. You can smell a battle looming from a mile away. In this case, though, DuPuis may either join you or join Quentin. If Varric is in the party, he responds in a way that can only be described as signature Varric -- awesome. Obviously, having a blood magic user really adds to your party's offensive punch, but on the other hand having to fight DuPuis along with everything else puts a real strain on your efforts. If DuPuis happens to join the fight in opposition of you, he'll need be the center of your attention.
Quentin begins the fight enveloped in an impenetrable barrier -- it can't be dispelled or anything. It's not like you don't have other things to worry about as you wait for the barrier to disappear. An army of undead comprised of Corpses, Abominations and the like, to be specific. Though nothing rattles your bones more than having to fight a Desire Demon that resembles his previous victims as well. Once you've dispatched Quentin's abominable creations, he will finally leave his protective bubble and open himself to murderous reprisal, though he'll actually perish very rapidly under pressure.
The rest of the quest deals with your and other people's response to recent events.