the demon king prophecy said that the next maou would come from a foreign land so iruma has the first criteria, and next the ring of solomon is actually disguised as the glutonous feeder ring so iruma has fulfilled another criteria. and yes i think there's something special about iruma apart from the fact that he's iruma, sullivan must have had a special reason for choosing iruma. i mean demons are naturally stronger than humans but iruma can keep up with their daily life and their extremes without suffering any physical or mental issues hence he's a special human to begin with cos if the prophecy just said any human sullivan would have taken any other human ╮( ̄▽ ̄)╭
i think sullivan didnt go search out for him specifically and that iruma just happened to be at the right place at the right time that fufilled the conditions that sullivan was looking for such as being strangely resilient. and iruma just so happened to be a door mat and cant say no and fufill the demon king conditions.
So I have few theories and thoughts...
There are few mysteries that haven't been answered in the manga so I have few theories about them
1. Why did Sullivan adopt Iruma
I think there has to be reason why Sullivan adopted Iruma besides his parents selling him. Even Opera was questioning him about it at some point.
2. And where is the Demon king
Is there possibility that these two things are connected?
So... My theory is that him adopting Iruma has something to do with the Demon king. Either the demon king is or was a human. Or Iruma is his relative or something. Or it could be both. Or if the demon king is dead like the the bad guy said, Iruma could be his reincarnation. And if the demon king was human it makes sense that he's dead if he has the humans lifespan... But there is also possibility that he was a human who turned into demon...
And there is whole thing with demon kings ring...
So many mysteries!!!
What do you think? (=・ω・=)