If memory serves, to start with a Demon ruler (as goblins do), a civ must have the Dark Tower site type, no RELIGION token, and VARIABLE_POSITIONS:ALL. I don't know if the demon ruler will come when the king would.... I think he would.
This system used to be more flexible, but in recent versions messing with it causes world gen crashes. (Ie, a Dark Tower civ with a regular religion crashes, as does a regular site type with no religion, as does a Dark Tower demon civ with pre-coded land holders, I think).
This conforts me in weird way: I was trying to understand why I couldn't keep Rhenaya's drows in their 34.11 Dark Fortresses in 43.05, I constantly had crashes in world generation. Changing it to a detailed cave "solved the issue" - but without solving this thread's problem and without solving my "translate with utter fidelity" problem.
On topic: could something be done to transform the dwarven king to into a demon somehow? Like in the way in which Boltgn's Succubus transofrm prisoners into other things. Succubus are also available in Meph's Masterwork.
I get that it is not the same as a society founded and always ruled by a demon but...
The alternative, if I understand correctly what D_E said, would be to add a separated entity of dwarves, which live in Dark Fortresses without Religion tokens and without any Position but "VARIABLE_POSITIONS:ALL". Call it Entity: DEMON__MOUNTAIN?
EDIT: I'm still the last post, better than doubleposting.
I tried it.
I removed RELIGION tags, substituting the positions with [VARIABLE_POSITIONS:ALL] and [SITE_VARIABLE_POSITIONS:ALL] and adding: [DEFAULT_SITE_TYPE:DARK_FORTRESS], [LIKES_SITE:DARK_FORTRESS] and [TOLERATES_SITE:DARK_FORTRESS].
I generated a quick pocket world with a dwarven civilization lead by a female moongoose demon and another one lead by a dwarf king from the beginning. "It works". I'd recommend changing some of the language symbols for the "demon" civilization to clarify what is what without having to look at the Legends file.
But embarking with the "demon" dwarves you find yourself without nobles at the embark. I did not play, just controlled that, I don't know if some form of that appears later.
It's up to you if playing without nobles is unacceptable or a wet dream, but that's something to notice.
Also, about the names: would there be a way to differentiate between two entities with the same ethnicity? Both the "normal" and "demon" dwarves here are called "dwarves" on embark and on Legends - the creature's name, not the entity's.
Would it be possible to differentiate that without copy/pasting one of the creatures into a different named species?
Cheers folks!
EDIT: D_E was correct again about the positions in a dark fortress site: adding them back causes crashes in world generation. Maybe we can try some investigation with [VARIABLE_POSITIONS:ALL] and [SITE_VARIABLE_POSITIONS:ALL] to see if it is one, the other or both of them needed.