If Illiya had grown up among the demon race, then he would never have said something like, "Eighteen is already an adult." Among all the races, only humans considered eighteen to be the age of adulthood. Therefore, the only two reasons the Demon King could confidently make such a statement were.
First, that he had grown up in the human world and had only now been brought back.
But that theory didn't seem likely. The demon race had a growth cycle much like the elves. Their growth cycle is extremely slow. The shortest ones still took at least a hundred or two hundred years; the longest, according to historical records, could take up to a thousand years.
Most importantly, body size was actually related to the growth stage.
And the growth period—for long-lived races—was the most unpredictable factor in their entire lifespan.
All things considered, it was highly unlikely that Anos had been raised in human society.
That left only one possibility.
