I remember this quest being buggy the first time I played a few years ago. Decided to replay DAI since I have a better PC now, and I've not yet played the Descent & Trespasser DLC. Anyway, I just completed this quest yesterday, so I'll list how I approached it, just in case in might help someone.
Received the quest "All New, Faded for Her" from Solas after talking to him at the Keep. I was already exploring the central & northern part of the Exaulted Plains.
Returning to the Plains, I went W-NW to destroy a Rift along the River. Made my way South along the river from that point, came to a small Ruin (which is an important part of another quest), defeated some Freeman fighters/archers. Solas made a brief statement: "We're getting close now, I thank you Inquisitor for helping me with this." (or something similar to that).
Continuing South along the river from the Ruins, I came to a small path/road where there were some bodies. Two of those I was able to interact with. A dead mage which had been killed by bandits, and another bandit body that had claw marks. Solas became upset with the claw marks. We continued a bit to the SW towards the River where we saw the Demon. Solas says a few things about Spirits & Demons.
We encounter the stupid mages who summoned the Spirit. Short conversation with them, Solas & myself. Solas says we must hurry and break the binding circle (or whatever it's called).
Battle begins with the pillars. Do not attack or allow your party to attack the Demon. Concentrate on destroying the pillars. Once destroyed, a short cut scene with the Spirit (no longer a Demon) and Solas. Solas will leave the party, and you will re-unite with him back at the Keep.
While there may not be any long reaching affects of this quest, you do gain major approval points with Solas at various points.
