Almost all powerful black magicians I know refer to Belial as mighty king, and others(demons) as kings.
What so special and exceptional about Belial?
Why is he so called a mighty king(not just a king)?
Is he the most powerful Demon in the infernal Empire?
Is he the quickest to action(when called on)?
Is he really the most terrifying demon(beast)?
Does he posses most secrets?
Itz Rachel Belial.
Who is Rachel? 
Is that supposed to be “Itz Ra’Cha Belial?”
All of those questions have subjective answers. He is a powerful spirit, yes, but as to the most powerful, no one can say that about any spirit, as the number of spirits in the multiverse is infinite and there is always something bigger.
Auto correct did it’s job there !!! 
It’s a sign of admiration and respect on they party, and an entirely personal choice. You could ask them why they call him that.
Me, I just call him Belial. I rarely use honorifs of any kind, I think they’re much too human and sound strange to me.
Wow I’m impressed, your asking the right questions. I think this is the beginning of your journey bud. Belial is touching you and reaching out to you. I think your journey with him has actually already begun.
one of my greatest challenge now is doubt
Maybe operator error 
The first time Lord Belial spoke to me, he told me to call him Lord Belial, specifically. I think it’s fascinating the way the terminology changes, like performing the same functions but using different programming languages. My assumption is that he is choosing a title based on your perception of the word itself, so for some it would be Lord, and for others King, based on the emotional impact the word has for that person. Just a guess tho