trial_by_combat_is_valid_trigger

trial_by_combat_is_valid_trigger = {
	# Calc some stuff that might invalidate elsewhere before the bout is sorted.
	trial_by_combat_shown_and_not_invalid_trigger = yes
	# Must have an imprisonment reason on the character.
	scope:puppet_or_actor = { has_imprisonment_reason = scope:recipient }
	# Scope:recipient must be able to fight or else have at least *one* designatable champion.
	scope:recipient = {
		trigger_if = {
			limit = {
				OR = {
					can_start_single_combat_eligibility_checks_trigger = no
					NOT = {
						can_be_combatant_based_on_gender_trigger = { ARMY_OWNER = scope:recipient }
					}
				}
			}
			custom_description = {
				text = "fp1_tbc_no_available_champions"
				OR = {
					any_knight = { eligible_knight_to_champion_trial_by_combat_trigger = yes }
					# Can use champion court position too if available
					AND = {
						employs_court_position = champion_court_position
						any_court_position_holder = {
							type = champion_court_position 
							eligible_knight_to_champion_trial_by_combat_trigger = yes #trigger checks for prowess, and while its not mandatory that the champ has a high value, we don't want to trap them into having to use their prowess 3 champ to defend their life
						}
					}
				}
			}
		}
	}
	# Scope:recipient must not be banned from single combat.
	scope:recipient = {
		custom_description = {
			text = "fp1_tbc_recipient_banned_from_combat"
			can_start_single_combat_banned_checks_trigger = no
		}
	}
	# Neither of the characters can be imprisoned.
	scope:puppet_or_actor = { is_imprisoned = no }
	scope:recipient = { is_imprisoned = no }
}

As written in game/common/scripted_triggers/01_fp1_scripted_triggers.txt.