In creating my campaign setting, I went well beyond looking at my copies of the Monster Manual and the Fiend Folio. I hauled my ass to YouTube and started looking for all the videos on the Nephilim and the Watchers that I could find. I turned on my scholar brain and began listening (and reading) for core concepts, something that Crazytown is lousy at (because the free stuff is glorified sizzle reel for their books and DVDs), and finally got enough to get working.
- "The gods" are the children of the Watchers, the Nephilim, sometimes with assistance from their divine parents. They are giants, of various sizes, and of varying supernatural power.
- As the Nephilim were never intended to exist, when slain their spirits remain in the world; these are the demons, and they long for physical existence more than anything else (hence all the possession) and hate Mankind more than anything else (hence all the targeting of men for oppression and possession).
- The exact relationship between the Watchers and the Adversary is left for me to decide, as none of the videos spell it out, but I can intuit the likely relationship.
- There is definite "purity/corruption" dynamic going on, as the Watchers are the source for all sorts of "corruption of the flesh" (i.e. creation of hybrids and other banned blendings of distinct beings). Unnatural blending of kinds, therefore, results in spiritual corruption (i.e. Face-Heel Turns) and always creates Always Evil monsters as a direct and immediate consequence.
This, right here, changes all of the assumptions about the supernatural flora and fauna of the setting. It makes reading the Monster Manual utterly useless, and it also makes cosmological assumptions just as useless because that won't be the same either. I can see the Mech Pilots getting nervous from here, as their encyclopedic lore knowledge no longer applies- and neither does the rules mechanics often tied to that lore.
This is how Dave Arneson, Gary Gygax, and M.A.R. Barker had to do it back in the day. They weren't reading generations of D&D books, each of which increasingly referred only to other D&D books, but instead taking all of their outside sources and finding ways to make fun gameplay out of it.
It also means that players can rely more on real-world weirdness that spending hours pouring over D&D manuals, which also means that players can stop relying so much on mastery of the rules to make their own fun and find satisfaction in gameplay. They can talk more in real world natural language, and less in game-specific mechanical jargon, and that makes the game (and the campaign) far easier to get into due to removing an unnecessary barrier to entry.
The game works best when players can come at it cold (no preparation) and stupid (no prior knowledge), and removing dependence upon any such things to become effective at the table is a good step for the Game Master to take. Creating your own setting, and setting up the campaign such that players automatically default to their men being as unknowing as they are, is the best way to go about making that work.