Bait and Switch (quest)

Acquisition
Make sure you have completed A Business Discussion, in Gamlen's house read the Bait and Switch letter at the writing desk to accept the quest.

Walkthrough
Travel to Lowtown at night to meet Anso the Contact, then head to the Abandoned House entrance in the northeast area of the map. If you have yet to complete the Sharp Little Pinpricks side quest, you will encounter the hostile Sharps Highwaymen here.

Inside the Abandoned House, open the south door and kill all the hostiles; further groups will spawn as the battle progresses, so be ready to move your ranged party members out of harm's way. The Street Thugs aren't tough but could trouble a low-level mage in numbers. Open the Smuggler's Cargo chest, then leave.

After the cut scene, the Tevinter forces will attack. The initial group will receive reinforcements, so try not to allow your party to become too spread out. Approach the steps to the west to be introduced to Fenris. Agree to help him during the dialogue that ensues, then travel to Hightown at night.

Meet Fenris in the Hightown Estates area, then enter Danarius's Manor. The elf will be locked into the party for the next section of the quest.

Fight your way through the numerous Shades until you reach a room in the northeast corner of the manor. Kill the enemies here and search loot piles to obtain the Magister's Key. Return to the large room with two staircases that lead to a mezzanine level, then save your progress and prepare your party for battle.

Unlock the Magister's Chambers to trigger the appearance of an Arcane Horror. A cut scene will ensue, and Fenris will leave the party. Collect everything of note then head outside, and one last cut scene and the quest is over.

Result
If you refuse to help Fenris in your first meeting, neglect to enlist him at the end of the quest or simply leave him standing outside Danarius's Manor when you depart Kirkwall for the Deep Roads Expedition, you will permanently lose the option to recruit him and undertake his future companion quests.