Okay, so this one may seem a little farfetched, but it seems like the most plausible option to me.
Basically, I think the Blood Emporer and his people might have been survivors of the Cataclysm, who managed to pool what few remaining resources they could find, gather together, and put themselves in a sort of stasis for thousands of years to wait out the worst of the horrors and re-emerged once things had stabilized and societies were re-built. When they emerged what they found was still primitive by their standards though.
This explains:
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Where they came from. The direction that their ships came from is well-known on the continent, yet nobody has yet found any populated lands in that direction. What is known is that they sailed from the general direction of the northern ice oceans… Great quantities of ice seem like they would be a great sacrifice/component in such a stasis spell.
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Why they established no contact or trade with their homeland, including never seeking revenge for any sort of hypothetical exile.
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Why they seemed to have deliberately hid all details of where they came from.
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Why they seem to have been able to greatly advance magic, and why they were so obsessed with that advancement. They would’ve been from a more advanced time, dropped into a backwards and primitive society, and while no small group of random individuals has all of the knowledge to run the entirety of an industrialized society, they would have enough of a basis to help compel fruitful research and likely would want to try to restore things to their level of advancement.
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Part of why they had so few ethical principles if they came from a more desperate/apocalyptic time.