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UPDATE: Theory seems debunked. I wrap that up here, but feel free to keep chatting about connections between III and XIV, or that beloved topic of the void.
Original post below.
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Work with me here, folks. If this is a house of cards, I want you to either concrete it, or bring it crashing down on me.
Here is my foundation.
We are told the Crystal Tower is the tomb of Xande, emperor of ancient Allag. But why would you make a tower your tomb? Probably because it became your tomb after you were using it for something else.
III tells us that Xande, from his base in the Crystal Tower, drained the power of the Crystals of Light. Now from an XIV perspective, that's a great reason to put your tower on top of the centrepoint of the planet's aether flow, Silvertear.
ARR has already shown us two of these crystals of light, and named them as such. It has also shown us a crystal of darkness, held by an ascian. III's lore speaks of the crystals of darkness existing in a counterbalance, which will call forth warriors of darkness when the light becomes overbearing, just as the crystals of light will do the opposite.
It is fitting that an ascian hold a crystal of darkness. Their motives revealed so far seem well aligned to returning the world to a state of darkness, as if they were cooperating with the Cloud of Darkness in her goal to plunge the world into shadow, a goal that she was manipulating Xande into achieving.
The world created by Xande in III greatly resembles what we know of the fourth Astral Era. In XIV, Allag empire had fallen. In III we hear of a recently fallen empire that had sought to use the power of the crystals of light in its own advancement - this sounds a lot like using the power of aether to drive powerful magitek.
Could the rise of the sorcerer Xande as allagan emperor and his manipulation by the Cloud of Darkness have led to the fall of his own empire in his greedy quest for immortality, thus bringing on the fourth umbral era of Hydaelyn?
If so, can we expect to see the floating continent that existed in that world? (Nym?)
((Hint: To bring down or cement this house of cards, try and focus on Bahamut - this is what needs to make or break the concept and I can't remember enough about his presence or fate in FFIII.))