{
  "type": "forge:add_spawns", // Required
  "biomes": "#minecraft:is_overworld", // Accepts a biome id, [list of biome ids], or #namespace:biome_tag
  "spawners":
  {
    "type": "irons_spellbooks:necromancer", // Type of mob to spawn
    "weight": 9, // int, spawn weighting
    "minCount": 1, // int, minimum pack size
    "maxCount": 1 // int, maximum pack size
  }
}