Upon the subject of demons, I have been putting together some content on the wiki outlining my structure of these monsters.
Starting with the creature itself, there is the Demon page. This concentrates on what a demon can do, with a quick introduction and then a discussion of their abilities. Wanting to give them powers closer to those I have read about in many an ancient text, I made their most important power the ability to possess others ~ and not once a day and not by the use of a magic jar spell. Certainly, very unpleasant, but I insist that monsters should not be weakened just because it will make players uncomfortable.
And this is why I've gotten rid of several things that the books tried to sell as "fun" but are, in fact, imposed handicaps on what should be a terrifying monster. I just don't understand. If the beholder does not have a special "amulet" that enables the beholder to be controlled just because the players have gotten a hold of it, why should a particular demon? If mind flayers and sphinxes don't have to worry about people knowing their names, why should a demon.
I understand that these things are supposed to be clever and adventure making, but it is really crappy, trope-driven adventure making, the sort of awful cliche that we're always seeing in TV supernatural series so bad writers can explain how a bunch of "good ol' boy" humans can get an edge on something that ought to be able to kill them outright. They're cheap, cheesy off-switches for monsters and they are inexcusably stupid.
I just don't see that characters should be able to kill demons at all; but I don't have an experience system that is based on killing anything, so it works out for me. At best, I expect a party to fight one off long enough to get it to teleport somewhere else ... which, let's admit it, is good enough.
Oh, and I also made a page on Demonic culture.
I have been playing with this concept that's based on arguments I've made in the past that the Gods are only as powerful as the belief that people have in them. I'm making up my mind to go one step beyond this ~ that the gods don't exist at all until they are invented by creatures on the prime material plane. This is what I was getting at with the Gehenna story.
It is always presumed that the gods must be much, much older than we are ~ but why? We invented the gods, didn't we? How does that necessarily change the freedom with which the gods act, why does it matter when they came into existence?
We conceive of things all the time that then get way out of our control. It's easy to imagine that if our thoughts were able to create a real god Zeus, that's going to get out of hand very quickly ~ particularly if we don't know our thoughts created him. We're just going to assume that he's been there all along and that we've "discovered" him.
There is a story that Zeus created the goddess of wisdom, Athena, directly from his head; "born from the head" will turn up the story if you search it on google. I've decided to call the process of giving birth to gods (and therefore to places within the outer planes) as "Thoughts Made Manifest." This is, without a doubt, the scariest idea that can be imagined, if we apply it to the actual creation of things simply because we invent them. But some readers will remember the old Star Trek episode that played havoc with the party on account of that.
I'm not saying that one character's thoughts will suddenly produce a god. That is not enough belief. But a thousand characters? Ten thousand characters? At some point, there is a tipping point reached and the belief becomes real. And this is the premise I intend to build my entire god-based universe upon.
Anyway, I hope the demon content is fun.