mpo_blood_brother_common_enemy_sub_trigger

mpo_blood_brother_common_enemy_sub_trigger = {
	NOR = {
		this = scope:recipient
		this = scope:actor
	}
	OR = {
		#Rival of common foe
		has_relation_rival = scope:recipient
		#Is at war with common foe, you're not involved
		AND = {
			NOT = { has_variable = rebel_leader_peasants }
			save_temporary_scope_as = root_temp
			any_character_war = {
				is_war_leader = scope:root_temp
				is_war_leader = scope:recipient
			}
		}
		#Is in a faction against common foe
		any_targeting_faction = {
			any_faction_member = {
				this = scope:recipient
			}
		}
		#common foe is in a faction against recipient
		AND = {
			is_independent_ruler = no
			save_temporary_scope_as = rival_temp
			liege ?= {
				any_targeting_faction = {
					any_faction_member = { this = scope:rival_temp }
				}
			}
		}
		#Recipient is title claimant against common foe
		any_held_title = {
			save_temporary_scope_as = title_temp
			scope:recipient = { has_claim_on = scope:title_temp }
			NOT = {
				scope:actor = { has_claim_on = scope:title_temp }
			}
		}
		#common foe is title claimant against recipient
		any_claim = {
			save_temporary_scope_as = claim_temp
			holder ?= scope:recipient
			NOT = {
				scope:actor = { has_claim_on = scope:claim_temp }
			}
		}
		#Has house feud with common foe's house
		AND = {
			exists = house.house_head
			exists = scope:recipient.house
			exists = scope:actor.house
			NOR = {
				house = scope:actor.house
				house = scope:recipient.house
				scope:actor.house = scope:recipient.house
			}
			house = {
				any_house_relation = {
					has_house_relation_level = feud
					any_relation_house = { this = scope:recipient.house }
				}
			}
		}
	}
}

As written in game/common/scripted_triggers/mpo_scripted_triggers.txt.