bg_fire_clean_up_and_invalidate_effect

bg_fire_clean_up_and_invalidate_effect = {
	remove_engaged_in_board_game_variables_effect = yes
	# We try to wing this at scope:bg_defender...
	if = {
		limit = {
			scope:bg_defender = { is_alive = yes }
		}
		scope:bg_defender = {
			trigger_event = { saved_event_id = scope:invalidation_event }
		}
	}
	# ... then scope:bg_attacker...
	else_if = {
		limit = {
			scope:bg_attacker = { is_alive = yes }
		}
		scope:bg_attacker = {
			trigger_event = { saved_event_id = scope:invalidation_event }
		}
	}
	# ... then scope:bg_initiator...
	else_if = {
		limit = {
			scope:bg_initiator = { is_alive = yes }
		}
		scope:bg_initiator = {
			trigger_event = { saved_event_id = scope:invalidation_event }
		}
	}
	# ... then we give up, as somehow *everyone* involved in the board game has died simultaneously.
}

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