Repentance

Acquisition

 * You must have access to 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). You can find the Starkhaven Longbow, a gift for Sebastian that can be wquipped, in the bedroom slightly north of the Waypoint marker. Before you continue, take a detour to the east section of the mansion for a amusing cutscene.

On arrival at the waypoint, you will encounter shades and desire demons. Kill them then 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 Revenanat and an Arcane Horror.

Keep going and you will find Allure,a desire demon, and Lady Harimann, who is a powerful blood mage. Try and go after the mage first so she can't summon more potent attacks.

After you have defeated them, return to the chantry and speak with Sebastian to end the quest.

Result

 * Sebastian joins as a permanent companion
 * You also unlock the Faith quest in Act 3.