Dragon Age 2 Quest Reference: Act 1 - Main Plot - Blackpowder Promise
Requires: Long Way Home
This quest is triggered when you're traveling through the Free Marches, you'll find Jarvis under attack, save him and the quest will start after you talk to him. Don't leave the Dead Man's Pass area before finding the Caste Treatise, you'll need it for the Caste Treatise and House Accounting quest and it's only available now. It's in the path on the right from where you came in on a body.
They'll be several battles before you finish. Once you're done, head over to the Docks to get your money. Keep in mind that you'll need to talk about payment to get any, he won't offer the money up front for the quest.
During this quest you'll unlock the Shepherding Wolves and Nice Crime You Have Around Here quests, the second of which can be triggered by other quests as well.