Background
Gascard DuPuis is an Orlesian blood mage of noble birth who now lives in Kirkwall. He is viewed by others as a reclusive man who rarely leaves his estate in Hightown. Several years ago, his sister was killed by a murderer who now roams in Kirkwall. He moved there to avenge her.
Involvement
In the quest Prime Suspect, the templar Emeric thinks that Gascard Dupuis is the man who kills women - and Gascard's own sister. When Hawke goes to search DuPuis' estate, they find him with a woman. That woman thinks Gascard wants to hurt her, but he explains to Hawke that, in fact, he wants a sample of her blood to protect her thanks to blood magic.
Result
Hawke can decide if whether or not he kills Gascard Dupuis.