Once you start act 2 in Dragon Age 2 you will also get a ton more side quests. Killing the Undying Xebeneck isn’t a quest in the traditional sense because you don’t have a marker for it. You have to search all over Kirkwall and the out laying areas. You have to find five evil tomes. Possible spoilers ahead.
Anyways for those of you who didn’t know killing the undying Xebeneck is a Trophy/achievement called Exorcist in Dragon Age 2. It has multiple parts but as long as you don’t go to the third chapter you will be able to go back if you miss one.
First thing you need to know about this is that when you find an Evil Tome you have to destroy them. Don’t try to get power or whatever out of it or you can’t summon the demon. Also after doing whatever with the Evil Tome you will have to fight some demons.
Also if you want Merril to be a friend instead of rival don’t bring her she doesn’t like you destroying the books it turns out. -10 per book destroyed.
Two of the Evil tomes are in Kirkwall. The first is in the Chantry. Take the stairs to the south and go into the room there. Or just click select and see it on the map since you are in the right area.
The second is in the Viscount’s Keep. Once you enter pretty much just head straight take the stairs up and go into the back room. You should see the tome on a bench.
Now we leave Kirkwall. Hit the Bone Pit first and enter the cave. Check your map once inside and find the Evil Tome deeper in the cave.
Now head to the Wounded Coast. Head north and take the third fork in the road and head left. Keep going until you find the Dank Cave. Check your map and find it in the cave.
The Last Evil Tome is in Sundermount. Go past the Dalish camp and stay on the western path. You will see a cave. Enter it and use your map to find the last one.
Alright with all the Evil Tomes destroyed its time to kill the undying Xebenkeck and get the Excorsist trophy/achievement.
Head to Dark Town and enter the Evil pit. You will find a Fell Tome. You can use this one to increase your abilities if you want. After doing that it will be time to kill the undying Xebeneck. She is just a desire demon who can summon more demons. Kill her to get the Excorsist trophy/achievement.
So there you have it killing the undying Xebeneck in Dragon Age 2. Not really hard just a tad time consuming for the Excorsist trophy/achievement. Good luck in the fight.