You can optionally report Leandra's disappearance to Moira in the Gallows, though doing so has no lasting effect.
There are two ways to get access to the Dark Foundry depending on whether Gascard is alive or not:
- If Gascard DuPuis is alive, you can find him in Darktown. Tell him to perform the ritual. Perhaps reflecting the gravity of the situation, Fenris and Anders will let this one instance of blood magic pass without comment or consequence. Sebastian, however, still disapproves Sebastian: rivalry (+5). If you turn down Gascard's offer to use blood magic, Sebastian approves of your decision Sebastian: friendship (+5) and Merrill disaproves Merrill: rivalry (+10) . It is still possible to accept Gascard's offer after turning him down. After the instant transition to the Dark Foundry, Gascard will accompany your party through the area.
- Certain dialogue choices made regarding Gascard in the quest Prime Suspect may result in Gascard's death at your hands or execution by the Templars, rendering this option unavailable. If you cannot find Gascard in Darktown, speak to Moira in the Gallows and she will reveal that DuPuis was executed. Gamlen then becomes your only aid.
- An alternative solution which will always be available is to meet Gamlen in Lowtown at night. Speak to the Urchin and either pay Varric: friendship (+5) or intimidate him Varric: rivalry (+5) to obtain information. Then follow the blood trail to the Dark Foundry entrance.
If Gascard still lives and you have performed the blood magic ritual to track down the location of the killer, Gascard reveals his ulterior motives and will betray you and take the side of the killer in the upcoming battle unless you are a diplomatic or aggressive Hawke, in which case you can talk him into helping you or intimidate him into assisting you.
When you arrive at the foundry, follow the blood trail to a trapdoor. As you battle through the area you will come across notes that begin to paint a picture of what's happening. This picture begins to become clear after the battle in the living quarters. At the end of the next hallway, a cut scene plays where you confront the killer. During the fight, the killer will summon swarms of abominations plus three possessed women in the form of desire demons, which you must destroy in order for him to lower the shield he generates around himself.
- If Varric is in your party, he has a very interesting reaction. Youtube Spoiler 
- Without Varric you can persuade Gascard to turn away from his current path via a diplomacy (Diplomatic personality only) option. If successful he will fight alongside you in the battle with Quentin. There is also a star option that becomes available with an aggressive personality that will convince Gascard to help you.
- If Gascard feels disinclined to assist Hawke, he will join Quentin for the final battle.
- The Gloves of the Overseer, Gloves of Enasalin, or Stonehammer Gauntlets are in a chest in the killer's lair, before the final encounter, depending on Hawke's class.
- In the Killer's Lair there is a letter to the killer from someone in the Circle of Magi, congratulating him on his discoveries and indicating that the writer has placed certain books on magic in a drop-off place for Quentin to obtain. The letter is signed "O." The identity of "O" may be revealed to be Orsino himself during the final quest of Act 3 if the player sides with Meredith.
- No matter which option you choose above (with Gascard's help or without), there is no way to save your mother from her unfortunate fate.
- If you recruited Gascard in Darktown, and he survives, you can talk to him after in Darktown. Bringing Aveline or Fenris to interrogate him proves enlightening. If you let him go here, Isabela: rivalry (+10), if you kill him Isabela: friendship (+10).
- If you are seeking the Supplier achievement, be sure to grab the Orichalcum in the Killer's Lair.
- If you bring both Aveline and Fenris, Aveline will take precedence for interrogating Gascard.
- If Hawke's sibling is still alive, Leandra will still say "I get to see Bethany/Carver (whichever was killed in the prologue) again and your father. But you'll be here alone." Technically this isn't true, as Hawke's other sibling is still alive (unless (s)he was killed during the Deep Roads) and Gamlen, Hawke's uncle, is still alive. Also what Leandra may mean when saying Hawke will be on his/her own, due to Bethany/Carver joining the Grey Wardens or the Circle/Templars, is that they would be unable to be with Hawke. One other family member would be Hawke's cousin and Gamlen's daughter, Charade, though Leandra never knew of her existence.
- The title of the quest may be a reference to the novel All That Remains by Patricia Cornwell, also involving the hunt for a serial killer.
- pc ps3 ps3 If you complete this quest by going to Gascard DuPuis in Darktown instead of going to Lowtown, you might still see Gamlen and the Urchin in Lowtown after the quest. Talking to the Urchin starts the dialogue and suggests that the quest has been started. It also gives rivalry points depending on your actions. The quest immediately says it's completed afterwards.
- pc Before talking to Leandra, DuPuis' disembodied voice may offer condolences despite your having killed him.