Normally this would prevent goblin civs entirely from appearing in the world, but if there are any UNIQUE_DEMONs defined in the raws, then only those will appear as leaders of gobbos.
Holy crap! This is an incredible tool. Most delightfully, it could be possible to flip the "demons" into a series of nature-themed monsters and make Elves the demon-worshippers!
An old thread response I found seems to indicate a few more limitations on that. I have not tested this for 47.01...
http://www.bay12forums.com/smf/index.php?topic=163667.msg7416609#msg7416609:
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.
Toady might have loosened the requirments when making [DEMON] and [UNIQUE_DEMON] available in the RAWs, so I guess this needs to be checked via trial and error in world generation. Even within the thread I linked it seems uncertain, you can have hard coded positions in addition to variable positions, it's just some may cause crashes when conflicting with the demon position.
Another old thread response with some relevant information:
http://www.bay12forums.com/smf/index.php?topic=110027.msg6920786#msg6920786Closest I know of at the moment is that a [SUPERNATURAL] creature that has taken a leadership role using its [POWER] token may write books about secrets if the secret and the creature both share a [SPHERE], which is [SPHERE:DEATH] for the default necromancy.
In a few of my own tests in 44.12, it seemed this technique was not working on individual custom castes added to standard races, but from other mods (in particular Spellcraft's Wizard race
http://www.bay12forums.com/smf/index.php?topic=149426.0), it seem to work on when used on entire creatures (and not individual castes)?
So you could make custom demons, give them each a mix of thematically appropriate spheres, then make spells for various spheres with various mixes of MUNDANE_TEACHING_POSSIBLE and MUNDANE_RECORDING_POSSIBLE, so that the resulting nature themed demon led civs then get various. And add various uncivilized creatures with the [POWER] and [SUPERNATURAL] tokens and various spheres to further go along with this theme.
One thing worth checking... if the elfs don't get driven out for immortality, will the elf necromancers still create undead lieutenants and/or experiments if given war/conquest to give them raw materials? I don't see what the tokens are for allowing experiments or setting what the experiments are (maybe not available in RAWs yet?), but if they do get made available in RAW some nature themed experiments would be fun. Likewise for nature themed intelligent undead...