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[...]thus hinting to the fact that the Demiurge had once been human (before the imprisonment of humankind)
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Hmm... interesting thought. I like it. But wasn't it so that the Demiurge created mankind in an image of himself? Maybe humans are Demiurge-clones :D
It is also mentioned that possibly the Demiurge created Astaroth as a image of himself to "complement" his being, in a way... Also it says somewhere that Astaroth claims to have created the Demiurge.
Most likely, Gunilla and Mikael wanted to leave these bits open-ended, to add some mysticism to the game.