[Spoilers for Main Campaign]
So I killed the Green Dragon and got the last(?) Jewel in the Crown. Journal now says "Return to Manacalon Ruin".
Upon entering the ruin, I get a massive boost to Masgarth relations (for literally walking into the area), and there's a brief cutscene where the Crown-bearer says something about "We Have a Problem." but it's not clear what the problem is. It's just the ruin as you left it, nothing seems to be different. Except now we can't sense any gems. But you thought you could sense one in a place you'd been to before, but now you can't because ..why?
The quest gets updated with "Return to the Council.".
What was the point of this journey? There are no decisions to make, no discoveries. Nothing. You literally turn around and leave again. Seems like there should have been _something_ to do in the ruins. Or just cut this waste of time out of the game and make it so that you can't sense any more gems after you get the last one.
No, I don't think you have forgotten anything.
This "useless" trip is only a narrative effect in my opinion. It is only there for the story.
I think it's there to remind you that the Soraks are looking for the crown and gems too, just like you saw when you first reached those ruins.
I rather think that this meaningless side trip is a result of cut content. There was probably a planned encounter (add-on area) that was simply removed from the game due to time/resource constraints.
The same is true of two minor quests involving spectral servants in Zhoron's mansion (Zhoron being the Master of Conjuration as well as a Green Dragon).
In fact, the final portion of the game is almost completely on rails. Unfortunately, both the narrative as well as the gameplay quality of the game suffers as a result.
I stand in the moonlit dawn, Free from clouds of attachment.
There is a stone gate at the end of the lava jumps on the second part of the map to the south, hit the wall and a secret compartment opens up and you get some treasure ( a quest item dwarven hammer, an unenchanted circlet and a gem for me) There is a fire elemental ambush once you get the items.
There is clearly a substantial amount of cut content in the latter 20% or so of the game. Besides that already mentioned, there are two areas in Copper, the mine and a guarded back road, that were obviously intended as quest areas. I'm hoping if they do new campaigns via DLC, they will add back some of this additional content to the original campaign, as a bonus.