Challenge to Duel

is_available = trigger_if = {
	limit = {
		is_ai = yes
	}
	any_relation = {
		type = rival
	}
	can_challenge_rival_to_single_combat_trigger = yes
	NOT = {
		has_trait = craven
	}
	is_imprisoned = no
	can_start_single_combat_trigger = yes
}
is_shown = scope:actor = {
	can_challenge_rival_to_single_combat_trigger = yes
}
OR = {
	scope:actor = {
		has_relation_rival = scope:recipient
	}
	AND = {
		scope:actor = {
			has_perk = stalwart_leader_perk
			government_has_flag = government_is_nomadic
		}
		scope:recipient = {
			government_has_flag = government_is_nomadic
			NOT = {
				is_close_family_of = scope:actor
			}
			can_be_combatant_based_on_gender_trigger = {
				ARMY_OWNER = scope:actor
			}
		}
	}
	AND = {
		scope:actor = {
			has_perk = stalwart_leader_perk
			culture ?= {
				has_cultural_parameter = stalwart_leader_non_admin_duels
			}
		}
		scope:recipient = {
			culture ?= {
				has_cultural_parameter = stalwart_leader_non_admin_duels
			}
			can_be_combatant_based_on_gender_trigger = {
				ARMY_OWNER = scope:actor
			}
		}
	}
}
scope:actor != scope:recipient

As written in game/common/character_interactions/00_perk_interactions.txt.

rival_single_combat_shown_and_not_invalid_trigger = yes
scope:recipient = {
	can_start_single_combat_trigger = yes
}
scope:recipient = {
	custom_description = {
		text = fp1_tbc_recipient_banned_from_combat
		can_start_single_combat_banned_checks_trigger = no
	}
}
scope:actor = {
	is_imprisoned = no
}
scope:recipient = {
	is_imprisoned = no
}
trigger_if = {
	limit = {
		scope:actor.culture = {
			has_cultural_parameter = stalwart_leader_non_admin_duels
		}
		scope:recipient.culture = {
			has_cultural_parameter = stalwart_leader_non_admin_duels
		}
	}
	NOR = {
		scope:actor = {
			government_has_mechanic = administrative
		}
		scope:recipient = {
			government_has_mechanic = administrative
		}
		scope:recipient = {
			is_liege_or_above_of = scope:actor
		}
		AND = {
			scope:recipient = {
				is_house_head = yes
			}
			exists = scope:actor.house
			scope:recipient.house ?= scope:actor.house
		}
	}
}
scope:actor = {
	custom_description = {
		text = character_interactions_hostile_actions_disabled_delay
		NOT = {
			has_character_flag = flag_hostile_actions_disabled_delay
		}
	}
}

As written in game/common/character_interactions/00_perk_interactions.txt.

on_send = scope:actor = {
	add_character_flag = {
		flag = flag_hostile_actions_disabled_delay
		days = 10
	}
}
on_accept = scope:actor = {
	custom_tooltip = challenge_to_single_combat_interaction.enter_non_lethal_bout.tt
	custom_tooltip = challenge_to_single_combat_interaction.victor_gets.tt
	custom_tooltip = challenge_to_single_combat_interaction.loser_gets.tt
}
configure_start_single_combat_effect = {
	SC_INITIATOR = scope:actor
	SC_ATTACKER = scope:actor
	SC_DEFENDER = scope:recipient
	FATALITY = no
	FIXED = no
	LOCALE = terrain_scope
	OUTPUT_EVENT = perk_interaction.0101
	INVALIDATION_EVENT = perk_interaction.0102
}
if = {
	limit = {
		scope:actor = {
			exists = scope:actor.house
			house = {
				OR = {
					has_house_unity_stage = harmonious
					has_house_unity_stage = friendly
				}
			}
		}
	}
	add_clan_unity_interaction_effect = {
		CHARACTER = scope:actor
		TARGET = scope:recipient
		VALUE = medium_unity_loss
		DESC = clan_unity_challenge_to_combat.desc
		REVERSE_NON_HOUSE_TARGET = no
	}
	custom_description_no_bullet = {
		text = active_unity_level_description_tt
	}
}
on_decline = scope:actor = {
	send_interface_toast = {
		type = event_toast_effect_bad
		title = challenge_to_single_combat_interaction.challenge_rejected.t
		left_icon = scope:recipient
		right_icon = scope:actor
		scope:recipient = {
			add_prestige = medium_prestige_loss
		}
		show_as_tooltip = {
			add_prestige = medium_prestige_value
		}
	}
	hidden_effect = {
		add_prestige_no_experience = medium_prestige_value
	}
	hidden_effect = {
		remove_interaction_cooldown = challenge_to_single_combat_interaction
	}
	if = {
		limit = {
			has_character_flag = flag_hostile_actions_disabled_delay
		}
		remove_character_flag = flag_hostile_actions_disabled_delay
	}
}

As written in game/common/character_interactions/00_perk_interactions.txt.