Duty (quest)

Acquisition

 * You must have access to the Premium Content entitled The Exiled Prince and have completed the quests of the same name.
 * Unlike many other companion quests, this can be completed during Act 1 or Act 2.

Walkthrough
Head towards the Chantry, and outside, right before the Chantry board a cutscene will come, it will show Sebastian putting a "hit" so to speak out on the people who murdered his family. Read it and accept the quest and head to the Docks(night), Sundermount, and Wounded Coast area.

You can do these areas in any order you choose, however it is easier if you head to that area when other quests lead you to it.

Once all 3 groups are dead, head back to the Chantry and talk to Sebastian to complete the quest.

Result
Completing Duty leads to the Repentance companion quest, which is available in Act 2, if you are already in Act 2, you may play it immediately.