diarch_coup_attempt_is_valid_showing_failure_only_trigger

diarch_coup_attempt_is_valid_showing_failure_only_trigger = {
	# Diarch must be able to act.
	diarch_cannot_be_disabled_trigger = { DIARCH = $DIARCH$ }
	# Plus standard checks.
	is_available_adult = yes
	# Filter this out in advance, else we risk trying to enter a duel later, only to find we're already engaged in one.
	custom_description = {
		text = ENGAGED_IN_SINGLE_COMBAT
		NOT = { exists = var:engaged_in_single_combat }
	}
	# Liege must have _something_ at the highest tier that isn't a HoF title.
	custom_tooltip = {
		text = diarch_attempt_to_overthrow_liege.tt.liege_cant_hold_only_hof_highest_title
		liege = {
			any_held_title = {
				tier = prev.highest_held_title_tier
				is_head_of_faith = no
			}
		}
	}
}

As written in game/common/scripted_triggers/00_diarchy_scripted_triggers.txt.