People seem to assume the Old Gods were either "keys" to the eluvians, or they were the ancient elven gods themselves. I have a theory that instead, they were the chosen of the ancient elven gods--immortal elves who were allowed to take the form of the "divine," the dragon.
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I think the Anchor may be holding the Veil in place.
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I think I tossed this one around in the past, but I believe that the head of the ancient elven pantheon was not Elgar'nan, which even the ancient myths seems to suggest, but rather Mythal.
I really like your theory of the old gods as the chosen of the elven pantheon. If Morrigan drinks from the Well of Sorrows, she gains the ability to shapeshift into a dragon, so it seems plausible. Also it's possible Fen'Harel and Ghilan'nain weren't part of the original pantheon and so may not have the dragon ability or chosen followers, which would explain why there are only 7 old gods instead of 9.
Regarding the Anchor - hearing that name for the Mark bugged me the whole game! Dagna describes it as a key, someone else says it's like a like needle. Those things seem to make sense because in the game you are opening/closing rifts (like a key) and physically entering the Fade (needle through the Veil). But an Anchor? Anchor to what exactly? Not sure I'm sold on the Veil itself but if Cory used the term, it's got to mean something interesting.
As for Mythal leading the pantheon, I'm on board with that. The codex about the traitor who took on the form of the gods says:
The sinner belongs to Dirthamen; he claims he took wings at the urging of Ghilan'nain, and begs protection from Mythal. She does not show him favor, and will let Elgar'nan judge him.
She will let Elgar'nan judge him? It's possible Mythal was known for being the more merciful of the two, but still. If she wasn't leading, it seems like they are on equal footing at least.
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Regarding trash items we can't bear to sell... I keep the Lover's Knots. So far I can't seem to get both the right and left half on a single character though.