minecraft_server_zombie/config/alexsmobs/mungus_spawns.json

{
  "biomes": [
    [
      {
        "type": "BIOME_DICT",
        "negate": false,
        "value": "overworld"
      },
      {
        "type": "BIOME_DICT",
        "negate": false,
        "value": "mushroom"
      },
      {
        "type": "BIOME_DICT",
        "negate": false,
        "value": "rare"
      }
    ],
    [
      {
        "type": "REGISTRY_NAME",
        "negate": false,
        "value": "terralith:mirage_isles"
      }
    ]
  ]
}