relic_war_valid_against_defender_trigger

relic_war_valid_against_defender_trigger = {
	any_character_artifact = {
		OR = {
			# Standard holy relics.
			AND = {
				is_holy_relic_for = scope:attacker.faith
				NOT = {
					is_holy_relic_for = scope:defender.faith
				}
			}
			# Struggle relics.
			## The Bells of Santiago de Compostela.
			AND = {
				exists = var:relic
				var:relic = flag:iberian_struggle
				scope:attacker = {
					any_character_struggle = { is_struggle_type = iberian_struggle }
					# Plus one of the valid using faiths.
					OR = {
						scope:attacker = {
							islamic_artifact_access_trigger = yes
						}
						scope:attacker = {
							christian_artifact_access_trigger = yes
						}
					}
				}
				scope:attacker.faith != scope:defender.faith
			}
		}
	}
}

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