Dragon Age 2 - Prime Suspect is a main plot quest, which continues on from the "First Sacrifice" quest from the first Act. In this walkthrough, players will infiltrate the estate of the titular prime suspect, Gascard Depuis, where after fighting his demonic minions, you will be given the choice of killing him or believing his story. The quest ends with a relatively easy fight against a group of Shades.

1. Prime Suspect is a quest you automatically pick up when you first visit your newly-acquired Estate at the start of Act 2. After the cutscene, go to the Gallows to speak with Emeric.
2. Agree to his proposal.
3. Go to Gascard Depuis' Estate in Hightown at night.
4. You will be ambushed a few times by large numbers of weak enemies on your way to the Estate.
5. Enter the Estate.
6. You will fight some Shades inside the estate. 
7. Continue on up the steps deeper into the building towards the quest marker.
8. You can decide to believe Gascard, or to kill him, but he is however telling the truth.
9. Head back out of the Estate to go see Emeric once again. You may encounter resistance on the way back depending on which path you take. Just keep in mind to focus your entire group on any powerful demons you encounter (such as a desire demon).
10. A different Templar, Moira, will tell you to go see Emeric. This is a trap, but don't worry.
11. Go and meet Emeric at the assigned spot in Lowtown at night.
12. You will speak with Moira once again after this. Here, if you didn't kill Gascard earlier,you can  decide to implicate him or to clear his name.