behavior_packs/vanilla_1.16/spawn_rules/ghast.json

{
  "format_version": "1.8.0",
  "minecraft:spawn_rules": {
    "description": {
      "identifier": "minecraft:ghast",
      "population_control": "monster"
    },

    "conditions": [
      {
        "minecraft:spawns_underground": {
        },

        "minecraft:difficulty_filter": {
          "min": "easy",
          "max": "hard"
        },

        "minecraft:weight": {
          "default": 40
        },
        
        "minecraft:density_limit": {
          "surface": 0,
          "underground": 2
        },
    
        "minecraft:biome_filter": { 
          "all_of": [
            {"test": "has_biome_tag", "operator":"==", "value": "nether"},
            {"test": "has_biome_tag", "operator":"==", "value": "spawn_ghast"}
          ]
        }
      }
    ]
  }
}