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unime: His lineage and inability to control his power.
Basically similar to Dan with his supposed part demon blood and unstable emotion stuffs.
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trilling:
Someone commented 'it's not his majesty's blood', so rather than the emperor's one night stand, it's more likely that he's a child of another royal who is dead (possibly from another 'trial' similar to this one)
In other word, he's adopted.
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ChI_I: I like to think that the 'issue' is that he's actually TOO powerful for the spells they're teaching him.
Like, it's like telling Godzilla to headpat a cat.
Can it be done? Sure, but compare to human doing it, it's gonna take a lot more care and effort not to squish it.