I can't get this new biome to generate in the nether. The biome shows up in the output folder and appears in the list when I list biomes, but it won't generate.

Tweaker.setScriptStage("BIOME_REGISTRY")
Tweaker.createBiome("basalt_plains","minecraft:hell")
Tweaker.setWorld(-1)

basaltP = forBiomes("biometweaker:basalt_plains")

basaltP.removeAllSpawns("CREATURE")
basaltP.removeAllSpawns("MONSTER")
basaltP.addSpawn("cofh.thermalfoundation.entity.monster.EntityBasalz", "MONSTER", 100, 4, 4)

basaltP.set("temperature", 2.0)
basaltP.set("genWeight", 1000)
basaltP.set("skyColor", 4873389)
basaltP.set("fogColor", 3160944)
basaltP.set("name", "Basalt Plains")
basaltP.set("topBlock", "quark:basalt")
basaltP.set("fillerBlock", "quark:basalt")

basaltP.addDicTypes("NETHER")

Tweaker.setScriptStage("FINISHED_LOAD")
basaltP.addToGeneration("DESERT", 2000)
basaltP.addToGeneration("WARM", 2000)
basaltP.addToGeneration("ICY", 2000)
basaltP.addToGeneration("COOL", 2000)

latest.log
I also tried doing the same with every other mod uninstalled but that didn't fix it.

That's not it either
`Tweaker.setScriptStage("BIOME_REGISTRY")
Tweaker.createBiome("basalt_plains","minecraft:hell")
Tweaker.setWorld(-1)

Tweaker.setScriptStage("FINISHED_LOAD")

basaltP = forBiomes("biometweaker:basalt_plains")

basaltP.set("temperature", 2.0)
basaltP.set("genWeight", 1000)
basaltP.set("skyColor", 4873389)
basaltP.set("fogColor", 3160944)
basaltP.set("name", "Basalt Plains")
basaltP.set("topBlock", "quark:basalt")
basaltP.set("fillerBlock", "quark:basalt")

basaltP.addDicTypes("NETHER")

basaltP.addToGeneration("DESERT", 2000)
basaltP.addToGeneration("WARM", 2000)
basaltP.addToGeneration("ICY", 2000)
basaltP.addToGeneration("COOL", 2000)`

Thanks for the reply, but that doesn't seem to be the problem. I removed the three lines regarding creature spawning and the result is the same.

It could be that you need to change the Script Stage back to FINISHED_LOAD (default) before accessing the biome with basaltP = forBiomes("biometweaker:basalt_plains"). I'm not sure how it works for that but if I guess it biomes are registered at the end of the biome registration meaning you can't access the biome through the resource location at the time.

basaltP.addSpawn("cofh.thermalfoundation.entity.monster.EntityBasalz", "MONSTER", 100, 4, 4) that isn't how you add spawns. You have to use the resource location like "cofh:basalz" although I'm not sure which they actually have, normally you find the resource location by advanced tooltips through F3+H for blocks and items or in your case you can use /ct entity_info to look it up. Any error in the script prevents it from executing and there is no error log!

The last thing I can think of is that you need to basaltP.addToGeneration("NETHER", 2000). In 1.12 there is only one biome for the Nether which resource location is minecraft:hell, so it could be that the overworld type don't even do anything. You could try removing Tweaker.setWorld(-1) or add an # in front of it to mark it as a comment and then try to find it in the overworld and if it exists, the generation types probably won't affect generation in the Nether.

It indeed spawns in the overworld. From what I understand, this mod can't be used to make changes in the nether, at least by itself. I found somewhat of a workaround tho:

I installed NetherEx and used it to get this biome to spawn. In saves/world file/config/netherex/nether_biomes/netherex there are four files which contain biome info. I made a copy of one of those and renamed it to "basalt_plains". Then I simply changed the first line to `"biome": "biometweaker:basalt_plains",. I don't know how much customization I can get out of this, but so far I was able to set blocks, entities and weight of the biome. So this looks like a promising method to make nether a proper dimension. Thanks again for your help.