I was confused a bit as well. She was talking about demons while I only saw a bunch of old elves and a magic swimming pool. It could be in reference to the figure coming out if the Well at the end (though I don't think Sera was in a position to see it).
That mission did confirm that elves had immortality since the sentinels had been alive all this time. Perhaps Sera is saying she believes this magical immortality is the work of demons? She also thinks that the Well itself is powered by demons so there's that as well.
Yes, maybe that's it. Or maybe she believes that the ancient elves themselves are in fact demons? Because that would explain their unnatural lifespan to her maybe.
I have a little problem with all of this though. Sera knows there's magic. She's afraid of it, but she acknowledges its existence. So why does she react to strongly, so surprised? Nothing about gods or ancient elves was actually proven. There's just a bunch of people who claim something, that's about it. And even if there had been prove, it's a world of magic. If you can accept magic, is the jump to "eternal living" really so hard? And since the gods were no gods but just powerful mages, as is suggested, she should rather be pleased.
I guess you are all right and it was something about the gods. But I just still don't think it was executed well, i.e. it doesn't make too much sense to me.
I'm a bit confused here as well, I know there were all kinds of revelations about the Elven pantheon in the Temple of Mythal, but I don't remember much of them in actual gameplay (that is not codices). I'm not sure what Sera was supposed to see that goes beyond the normal statutes and magic you see in in any Thedas ruins.
I agree that her reaction is supposed to be to proof of the Elven gods being legit though.
Yes, this is what I mean. There is not much to see. There's statues, a temple, a few guys who don't actually introduce themselves and our party only assumes it's ancient elves. And then they speak about gods and how they were in fact no actual "gods" or, when it comes to Mythal and Fen'Harel, how they were interpreted wrongly. That should not impress Sera much because she did not believe in Dalish lore to begin with.
The well might be something, but the issue is that somebody does drink from it, and it is somebody she knows. So there is no actual "need" to react so strongly. And the inquisitor does not seem to be part of her problem because she does not attack him/her implying s/he was the one lying (in case s/he drank from the well).
They've ignored more terminal conditions of characters before. Though I wouldn't expect it.
And in the end you can always make a DLC like Leliana's Song where you simply play Sera herself. I would love that.