All That Remains

Acquisition

 * Complete Prime Suspect and Offered and Lost

Walkthrough
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 Foundary:
 * If Gascard is alive, you can find him in Darktown. Tell him to preform the ritual. Perhaps reflecting the gravity of the situation, companions who usually respond bad to blood magic, will let this once instance pass without comment or consequence. After the instant transition to the Fark Foundry, Gascard will accompany your party through the area.


 * An alternative solution(and the only available path if Gascard was killed during Prime Suspect) is to meet Gamlen in Lowtown at night. Speak to the Urchin and either pay or intimidate him to obtain information. Follow the blood trail to the Dark Foundry entrance.

Result
If Gascard is with you in the Killer's Lair for the meeting with Quentin, there are three possible outcomes:
 * If Varric is in your party, he has a very interesting reaction.
 * Without Varric you can persuade Gascard to turn away from his current path via a diplomacy option. If successful he will fight alongside you in the battle with Quentin.
 * If Gascard feels disinclined to assist Hawke, he will join Quentin for the final battle. In this instance target him and kill him first, you do not want him alive when the first desire demon spawns.

Unlocks Following the Qun and The Captain's Condolences companion quests.