Dragon Age 2 Quest Reference: Act 1 - Secondary Quests - Bait and Switch
Requirements: A Business Discussion
This quest has quite a few battles. Since some low level teams might have a problem finishing it, it's a good idea to wait until you have a few levels under your belt before trying it.
Start this quest by reading the Bait and Switch letter in the Writing Desk at Gamlen's. The letter will be from either the Smugglers or the Mercenaries depending on who you opted to help in the Prelude. Go to Lowtown at night and talk to Anso. Then follow your minimap to the Abandoned House in The Alienage. Kill the Highwaymen if you haven't already done the Sharp Little Pinpricks quest, it will save you time later.
You're in for a big fight after entering the house, so it's a good idea to save first. The chest is empty, don't bother wondering why. Just speak to Fenris after the battle and say you'll help him. Then go to Hightown at night and meet Fenris in Hightown Estates. Enter Danarius's Manor and follow your minimap to the room containing the Magister's Key. Don't forget to loot enemies along the way.
Save before going back the way you came, particularly if you're still at a low level. Return to Hightown after looting everything and speaking with Fenris. You need to speak with him outside Danarius's Manor to finish the related Fenris Recruited quest, which basically just consists of talking to him. If you don't recruit him during Act 1, or refuse to help him, there are no other opportunities to get him in your party. All future companion quests from Fenris are obviously dependent on this quest.