All That Remains

All That Remains is a Main Plot quest in Dragon Age II during Act II revolving around the disappearance of Hawke's mother, Leandra.

Acquisition

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:

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 Dark Foundry, Gascard will accompany your party through the area.
- 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.

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 Varric: rivalry (+5) to obtain information. Then follow the blood trail to the Dark Foundry entrance.

If Gascard DuPuis 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.

The Gloves of the Overseer, Gloves of Enasalin, or (possibly) other class-appropriate set gloves are in a chest in the killer's lair, before the final encounter.