Repentance

Acquisition
Note that this quest is not available until Act 2, even if Duty is completed during Act 1.
 * You must have access to the The Exiled Prince downloadable content.
 * Complete Duty

Walkthrough
Speak to Sebastian in the Chantry and agree to help. (There will be a funny exchange with Varric if the humorous answer("Don't let me stop you") is chosen in the first dialogue) 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 through 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 and a complex chest. If Isabela is in your party, she will have some amusing comments.

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. After the battle, go up the stairs to the west to find the Codex entry: The Demon's Gift and a complex treasure chest. Before you enter the final room, don't forget to pick up 4 (exact amount varies with level) from some rubble in the corner.

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 Harimann:

Choice 1:
 * "I won't betray my friends"
 * "I do like power"
 * "Die, fiend!" (Attack)
 * "Die, fiend!" (Attack)
 * "Die, fiend!" (Attack)
 * "Die, fiend!" (Attack)

Choice 2 (If you did not attack Allure):
 * "That's a good point."
 * "Don't listen to her..."
 * "Die, fiend!"
 * "Don't listen to her..."
 * "Die, fiend!"
 * "Die, fiend!"
 * "Die, fiend!"
 * "Die, fiend!"
 * "Die, fiend!"
 * "Die, fiend!"

Dialog with Lady Harimann's daughter:
 * Diplomatic
 * Sarcastic/Aggressive
 * Sarcastic/Aggressive

During the Final conversation back in the chantry: Other dialogue choices here seem to have no impact.
 * "That's a good question"/"What do you want to do?"/"Listen to your heart"
 * "You must take back your land."
 * "You must take back your land."

Result

 * Sebastian joins as a permanent companion.
 * If the item was picked up, the companion quest The Starkhaven Longbow will activate.

Rewards

 * Sebastian's armor upgrade - Mail Undertunic (+2% Critical Chance)
 * The following items:
 * - loot Desire Demon
 * - loot Allure's corpse
 * 11 from Sebastian

Bugs

 * PC 1.04: Before the conversation with Allure & Lady Harimann the screen goes black and the "Loading" message pops up. To proceed with the quest simply press Esc as soon as you see the message.