Before we get too far into this, let’s do a few basic tips. For this battle, a balanced team should have no trouble. My warden and Sten could both hack their way through most of the groups while Leliana did backstabs. Morrigan’s Mind Blast along with her general magic will work wonders on keeping the enemy stunned and hurt.

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Go upstairs and walk down the hallway first. Don’t go into the room next to the start, since that will trigger a big fight. We might as well hit the two groups separately. Go down the hall and hit the corpses in the hallway. There’s not a trick to this- just take them out. You’ll probably trigger the next group of corpses in the next room. Grind through them too.
Cut back and go into the big room that we passed. There are about five shades inside this room. They’re a little tough, but we should still be able to take them out. They’ll die just like the corpses if you keep your wits about you. Check the book on the podium to get a new codex entry.
Head back out to the hallway. There’s a locked armory here, if your rogue can pick the lock, then you can get a little loot. Go down the next part of the hallway when you’re ready and open the door. You’ll activate a whole bunch of walking corpses and special devouring corpses. These guys are a little tougher, so focus on them if they show up. If you’re healed up, then you can use the doorway as a bit of a choke point.
Once this group is down, you can go a bit further. There are six Mabari hounds in the side room to the left, along with a charred body and some loot. These guys are pretty tough, since they know how to use overwhelm. This will immobilize and probably kill whoever they grab. If you’re having trouble with this part, you might want to skip it and hit it later.
The next hallway has a few rooms down the sides. You can kill the three corpses in the first room on the left. The next two sets are a little out of your hands, though. If you hit the fire trap just past the first room, the two other rooms should dump out their groups of corpses. The trap itself should barely scratch a warrior. The corpses are a little tough, but you should be able to beat it after a shot or two. The tactics are still the same.
The important room is at the back left corner. There’s a small room with a closed door. Open it up to find Valena, the blacksmith’s daughter. It turns out that she is alive. She’ll make a run for it.