All That Remains Quest Guide.
There is no way to change the final outcome of this quest. There are few different options along the way that will change how the story unfolds, but the end remains the same.
You must complete both Prime Suspect and Offered and Lost in Act II to receive this quest.
First you must return to Hawkke's estate and learn that Leandra is missing.
You can report Leandra's disappearance to Moira in the Gallows. (this has no over all effect on the quest)
There are two ways to get access to the Dark Foundry:
Certain dialogue choices may result in Gascard's 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.
If Gascard is alive, you can find him in Darktown. Tell him to perform the ritual.
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. Then follow the blood trail to the Dark Foundry entrance.
In the Dark Foundry
If Gascard DuPuis is with you in the Killer's Lair for the meeting with Quentin, there are three possible outcomes:
1. If Varric is in your party, he has a very interesting reaction.
2. 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.
3. 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.
Completing this quest will unlocks Following the Qun and The Captain's Condolences companion quests.
Choices in this quest line that effect companion friendship/Rivalry
Taking Gascard offer to perform the ritual +5 Rivalry Sebastian
Refusing Gascards offer +5 friendship Sebastian +10 Rivalry Merril.
Intimidating the Urchin +5 Rivalry with Verric.
For more information check out the wiki