In Faith&Flame there is an optional storyline on a Faerie becoming infernally corrupted and turning into a Demon. There was a similar development of a former guardian of place in the old Black Death scenario. (However, this was a 3ed scenario with the power realms less clearly defined. The former role of the entity was mentioned being divine. With the current ruleset it should be Faerie or Magical IMO.)
I think the concept makes for great storylines. Pagan gods turning into infernal False Gods--looks good! Creatures of Magic and Faerie being assimilated into Devine and Infernal realms are also perfectly fitting a theme of old pagan wonder and magic fainting from the world with the rise of Christianity.
Unfortunately I have a few problems here.
As a minor complaint on the concept in FF, the described Faerie was a formerly powerful pagan god. In my opinion it should not be an ordinary Faerie but a Faerie daimon, sending only aspects to mundane world. And I don't think that fits the described background story very well. But there is a much bigger problem.
According to RoP:tI Faeries are not capable of true evil, being amoral and not immoral. So, it is hard to see how a Faerie can truly become infernally corrupted. Worse, Faeries do not have a soul while Demons do. I guess a soul is necessary to become truly corrupted. Owning a immortal soul also guarantees eternal suffering in the end. A soulless Faerie-Demon would escape this fate when destroyed. And I don't think that a Faerie should be able to gain a soul by becoming corrupted (unless one thinks that the corruption is due to a divine act, as creating a soul is divine). Am I somehow wrong here?
On the other hand there are probably creatures with infernal (and divine) might that lack a soul. For example I guess corrupted beasts don't own one. However, I won't call these infernal creatures demons and introducing corrupted-Faeries-that-are-no-demons as another kind of infernal creatures seem to suck the life out of the original idea.
I am not sure about jinn. They come in all four realm flavours. Do magic and faerie jinn lack a soul, while divine and infernal ones have one? Or are they all the same?
And then, becoming immortal by gaining faerie might (or magical one) should change one into a Faerie (or a Magical being) still owning a soul. (Loosing ones soul in the process would make it much less attractive, doesn't it?) In this case there are a few souled Faeries. However, assuming only these special Faeries can be corrupted feels like a rather ugly and purely technical solution.
As a way out one might assume that every being has a soul. Refusing to acknowledge the souls of beasts, Faeries and magical creatures is just Christian propaganda (or simply a theological error). Some pagan believes are nearer to the truth then.
Would love to read your ideas on this.