I have a similar yet different hypothesis.
Do you remember the chapter that showed Claude's childhood? At the end of that chapter where Claude's older brother was looking at the wall as if mesmerised by something, I think that darkness might have been Aeternitas, the brat that Lucas denied to teach magic to.
From what I remember of that chapter Claude and his older brother had a really good relationship, Anastasius seemed to be taking and playing happily with Claude.
So I believe that Anastasius has been possesed by Aeternitas since childhood, slowly but surely Anastasius started to change from that sweet boy we saw in that chapter to the villain we see now.
Well that's just my opinion though~
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That would explain why he wants to kill Lucas so bad, Claude’s brother has no business with Lucas but the previous emperor was rejected by Lucas
Aaah That's true! I'm starting to feel sorry for Anastasius.. ╥﹏╥
Theory!
Maybe he and the emperor had a contract and he took over the dead emperor's body and came back to do the emperors revenge and maybe this guy used to be a disciple or an acquaintance of lucas