In your custom solar system give the planet you want the modifier(s) on a flag so it can be recognized.


moon = {
	name = "Sadaies"
	class = "pc_subarctic"
	size = 12
	orbit_distance = 8
	orbit_angle = 40
	has_ring = no

	init_effect = {
		set_planet_flag = is_sadaies_flag 						# this is the flag
		add_modifier = { modifier = kaiju_depths days=-1 }
	}
}





Then we create an on_action script (lets call it " sadaies.1 "

In the on_action folder/file look for on_game_start and add the above on action


on_game_start = {
	events = {
		game_start.1
		game_start.2
		game_start.3
		game_start.4
		game_start.5
		sadaies.1							# right here
	}
}





Then we create the actual event script that will run when the game starts. It will go trough every planet on the solar system and see if it finds a planet with the " is_sadaies_flag "


stellaris/events
Make a zz_on_game_start_event.txt or name it whatever you want




namespace = sadaies					#make sure this is in the filt at the top !!

### On game start, look for any planet that has the planet flag "is_sadaies_flag"
event = {
	id = sadaies.1
	hide_window = yes
	is_triggered_only = yes

	immediate = {
		every_planet = {
			limit = {
				has_planet_flag = is_sadaies_flag
			}
			add_modifier = { modifier = kaiju_depths days = -1 }
		}
	}
}