This is a fairly quick quest that you should be able to do while you're in the Deep Roads. It will start off once you find a small bloody sack with various dismembered body parts. There are three of these and each of them have a note that gets added to your codex.
The first one is Aeducan Thaig. If you're following Prince Bhelen's quest line then you will have found this while you searched the area for Lord Dace. If not, then you need to go there once the Deep Roads open. It's at the back of the first large cavern filled with darkspawn.
The other two are in Caridin's Cross. You should find the first after you kill a group of genlocks. Remember to hold down TAB to help find it. The second one is in the second area filled with deep stalkers. Note that it is not the area where you fight a matriarch. You will only fight normal deep stalkers here.
Once you have all three bags, you need to go to the altar of sundering in Ortan Thaig. It's marked on your map and it's part of the ruined city. You'll have to kill a golem and some spirits before you get to it though. Once there save and then activate the altar to reassemble the demon.
It's basically another pride demon. He looks like what Uldred was, for reference. He's not as strong though. You have the choice to either free him and demand a reward or fight him. If you free him, you'll get 25 sovereigns. The strange thing is that no one in your party should really care. I know that Morrigan, Sten, and Leliana are apathetic to it all.
If you fight him, you'll need a little luck. He can cast crushing prison on your tank and basically take him out of the fight. You have to hope that he sticks to his main attack, which is some general slashing and an area of effect spell that causes flame or ice damage to those in a circle around him. It's rough on your fighters, but your mage should be in a safe spot and ready to heal them.
Backstabs work well and magic seems to have a good effect. He can be stunned and briefly frozen, so use that to your advantage. Other than that, it's basically just another fight where you have to stay alive and wear him down.
Your only reward for killing him is the roleplaying value and about 100 XP. It's probably a better deal just to free him and pocket the money.