rival_single_combat_shown_and_not_invalid_trigger

rival_single_combat_shown_and_not_invalid_trigger = {
	scope:actor = {
		# Scope:actor must be able to fight personally.
		can_start_single_combat_trigger = yes
		# Scope:actor cannot challenge someone they have a ceasefire against.
		NOT = { has_truce = scope:recipient }
		# Scope:actor *definitely* cannot challenge someone they're at war with.
		NOT = { is_at_war_with = scope:recipient }
	}
}

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