fallback_move_to_pool_effect

fallback_move_to_pool_effect = {
	save_temporary_scope_as = mover
	if = {
		limit = { exists = host }
		set_variable = {
			name = last_visited_ruler
			value = this.host
			days = 1825
		}
	}
	else = { #I.e. if we're in the pool
		set_variable = {
			name = last_visited_ruler
			value = dummy_female #Because you must have a character value to compare it with another
			days = 1825
		}
	}

	select_fallback_pool_to_move_to_effect = yes

	every_traveling_family_member = {
		limit = {
			NOT = { is_in_pool_at = scope:destination }
		}
		move_to_pool_at = scope:destination
	}
}

As written in game/common/scripted_effects/00_courtier_guest_management_effects.txt.