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") or the diplomatic answer("Nothing bad, I hope") 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 (30 Cunning to open, 150 xp/chest).
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 or Varric is in your party, they 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 dialogue with the daughter.
Return to the Chantry and speak to Sebastian to complete the quest.
During the conversation with Allure & Lady Harimann:
Choice 1:
"I wouldn't betray my friends"
Anders: friendship (+5)
"I do like power"
Fenris: rivalry (+5)
Anders: rivalry (+5)
"Die, fiend!" (Attack)
Anders: friendship (+5)
Sebastian: friendship (+10)
Aveline: friendship (+5)
Choice 2 (If you did not attack Allure):
"That's a good point."
Sebastian: friendship (+15)
Varric: friendship (+5)
Aveline: rivalry (+5)
"Don't listen to her..."
Merrill: friendship (+5)
Varric: friendship (+5)
Sebastian: rivalry (+15)
Aveline: rivalry (+5)
"Die, fiend!"
Anders: friendship (+5)
Sebastian: friendship (+10)
Aveline: friendship (+5)
Note: To gain Merrill's and Varric's friendship points in 'choice 2', one must select "I wouldn't betray my friends" in 'Choice 1' instead of "I do like power".
Dialog with Lady Harimann's daughter:
Diplomatic
Sebastian: friendship (+5)
Sarcastic/Aggressive
Sebastian: rivalry (+5)
During the Final conversation back in the chantry:
"That's a good question"/"What do you want to do?"/"Listen to your heart"
Sebastian: friendship (+10)
"You must take back your land."
Sebastian: rivalry (+10)
Other dialogue choices here seem to have no impact.
Sebastian's cunning will likely not be high enough to open the locked chests within the house or ruins. The building remains open throughout Act 2 however, and it is possible to return later to empty them.
pc (1.04) Before and after the conversation with Allure and 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.