Repentance

Acquisition
Note that this quest is not available until Act II, even if Duty is completed during Act I.
 * You must have access to the The Exiled Prince downloadable content.
 * Complete Duty

Walkthrough
Speak to Sebastian in the Chantry and agree to help, then travel to Hightown and enter the Harimann Estate. Sebastian will be locked into the party from this point forward. There are three chests with complex locks in this quest.

Your journey though the mansion will pass without incident until you reach a cutscene. Collect the Mail Undertunic from the nearby chest (armor upgrade for Sebastian). Before you continue, take a detour to the east section of the mansion for a amusing cutscene.

As you approach the waypoint marker, a cutscene and battle with shades and a Desire Demon will follow. The Starkhaven Longbow can be picked up off a nearby dead Flint Company mercenary. Picking it up activates the companion quest The Starkhaven Longbow.

Enter the Underground passage to continue the quest. The first battle features a rage demon and numerous shades. Your next fight is with a Revenant, and a selection of corpses. Then after them, two more rage demons await, and beyond them is a Revenant and an Arcane Horror.

Keep going and you will find Allure, a desire demon, and Lady Harimann, who is a powerful blood mage. After some dialog, you will have to defeat them. They will be joined by several shades.

Return to the estate via the nearby exit. You will have a bit of dialog with the daughter.

Return to the chantry and speak to Sebastian to complete the quest.

Friendship/Rivalry
During the conversation with Allure & Lady Harrimann: Dialog with Lady Harrimann's daughter:
 * and if Hawke cuts the conversation with Allure short by attacking her.
 * , and, and  if the player chooses the dialogue option, "That's a good point."
 * ,, , and , if the player chooses the dialogue option "Don't listen to her..."
 * Diplomatic.
 * Sarcastic/Aggressive

Result

 * Sebastian joins as a permanent companion
 * Companion quest, Faith, will be available during Act 3.
 * If the item was picked up, the companion quest The Starkhaven Longbow will activate

Rewards

 * Allure's Crook - loot Allure's corpse
 * 11 from Sebastian
 * Presumably, completing this quest should unlock the "Avenged" achievement, but appears to be currently bugged (as of 25/03/11, Confirmed 360, Unconfirmed PS3/PC)