Light Spoilers for the original Fairy Tail manga. Not 100 years quest as far as I know (I haven't read it)
Basically Natsu becoming a demon is a sort of 'bad end' in universe that has some possibility to it, so I've twisted that for this scenario where he winds up being a perfect vessel for demonic power thanks to the same reasons those bad ends could happen.
I swung back and forth on if I wanted this to have more of a comedic tone but I wound up taking it a little more seriously. Though the evil demon summoners still do come across a bit lame, though in this case that makes the defeat more embarassing.
I had a version where Natsu kept his normal skin and eyes but I decided this looked better, with that version coming across more as a weird cosplay.