Pretty much everything except the djinn are manga-original, and even they attack at a different point in the story. Must say, I do appreciate them changing up the story like this… helps me be hopeful it won't turn into the same schlock.
I mean djinn are basically just islamic demons so...
That being said it's essentially a really strong djinn vs the former demon king, think the odds are stacked against him.
Author sure put all the effort into picking those djinn names. Though I suppose they probably sound cool and exotic and whatnot when you're Japanese and also chuuni as fuck. Meanwhile they'd probably laugh at some dopey English author who made themselves a character with a fake, kludged together "Japanese" name like Kenkami Kuroryu that doesn't actually make sense.
I'm not sure why the author bothered leaving the whole: "Are you going to chase me away for these magic stones?!" bit on a cliffhanger. We all know very well that the MC would never do something like that. As for the Djinn? Kinda hoping that the demon king doesn't job and actually can put up a fight.