IMR6/minecraft/config/alexsmobs/bone_serpent_spawns.json

{
  "biomes": [
    [
      {
        "type": "BIOME_TAG",
        "negate": true,
        "value": "forge:no_default_monsters"
      },
      {
        "type": "BIOME_TAG",
        "negate": false,
        "value": "minecraft:is_nether"
      }
    ]
  ]
}