It's one of those quests that is sensitive to how you approach it for it to work correctly. After you receive the quest and head to the Exalted Plains, start from that first camp and head West to slightly NW until you come to the River. I usually aim for a Rift that needs closed (NOT the one North of the bridge, there is another Rift about mid point on the River).
From that point head South along the River. You will come to a small ruin where you will have a small battle w/ bandits. The ruin itself is an important part of another quest, but only as a progress marker for your current quest. After that battle Solas will say something to the effect of: "We're getting close now, thank you Inquisitor for helping me with this".
Proceed South until you see a road/path on your left. This, unfortunately is very important, and the reason you should not approach this quest from the Elven camp, because you would likely miss it. Anyway, there are some bodies in the road that you need to inspect. You will inspect 2 of them. One is killed by arrows, but another has claw marks. Solas becomes upset at this discovery because it is from the Demon form of his Spirit-friend. A short conversation ensues.
Head SW to some pillars that you can see and you will encounter the mages that summoned the Demon. A 3 way conversation ensues between the Inquisitor/Solas/and the mages. Answer wisely if Solas is your friend.
Go to the pillars where the Demon is and destroy the 5 pillars. Do Not attack the Demon. When he attacks someone in your party have them move around instead of attacking him. Concentrate on destroying all 5 of those pillars. Once the pillars are destroyed, a short cut scene will happen with Solas and the Spirit (no longer a Demon). After that a short conversation and Solas will leave the party, rejoining at Skyhold.
You can earn some major points with Solas on this quest if you choose the right options.
I liked this quest, it's just unfortunate that those corpses are so closely tied to being able to complete it.
