Creature Type:

"OVERGHAST": {
	"Spawn Rate": 1,
	"Spawn Cap": 15,
	"Chunk Spawn Chance": 0.0,
	"Spawn Medium": "air",
	"Iterations Per Chunk": 3,
	"Iterations Per Pack": 4,
	"Spawn Tag": "!sky()&&lgcy.dimension(0)&&obj.light>7&&wrld.timeofDay(13000)||lgcy.dimension(1)",
	"Default Biome Cap": -1,
	"Biome Caps": {}
    }

	I know its a bit redundant with the spawn tags in the config below, but just in case...

SpecialMobs config:

"SpecialMobs.UnholyGhast": {
      "Type-Enabled": "OVERGHAST-true",
	"Spawn Operand": "OR",
	"Spawn Tag": "!sky()lgcy.dimension(0)&&obj.light>7&&wrld.timeofDay(13000)||lgcy.dimension(1)",
	"Despawn Tags": "obj.light<8",
      "Contents": [
        "SpecialMobs.UnholyGhast"
      ]
    }
	^ This applies to every Ghast mob in the config.