atm9/client/config/alexsmobs/mungus_spawns.json

{
  "biomes": [
    [
      {
        "type": "BIOME_TAG",
        "negate": false,
        "value": "minecraft:is_overworld"
      },
      {
        "type": "BIOME_TAG",
        "negate": false,
        "value": "forge:is_mushroom"
      },
      {
        "type": "BIOME_TAG",
        "negate": false,
        "value": "forge:is_rare"
      }
    ],
    [
      {
        "type": "REGISTRY_NAME",
        "negate": false,
        "value": "terralith:mirage_isles"
      }
    ]
  ]
}