set_elder_relation_effect

set_elder_relation_effect = {
	$DISCIPLE$ = {
		save_scope_as = new_disciple
		random_relation = {
			type = elder
			save_scope_as = old_elder
		}
	}
	$ELDER$ = {
		save_scope_as = new_elder
	}
	if = {
		limit = {
			# no circular relations please
			NOT = {
				scope:new_disciple = {
					has_relation_elder = scope:new_elder
				}
			}
			# Disciple does not have a higher merit rank than the elder
			# The elder can have more disciples
			scope:new_elder = {
				eligible_for_elder_trigger = {
					DISCIPLE = scope:new_disciple
				}
			}
		}
		scope:new_disciple = {
			# in case you already had an elder, remove the relation and send them a letter
			if = {
				limit = {
					exists = scope:old_elder
					# Guard against the disburse loop picking the same character as both old_elder and new_disciple, which would make add_opinion target self.
					scope:old_elder != scope:new_disciple
				}
				remove_relation_elder = scope:old_elder
				save_scope_as = actor #for loc in tgp_interaction_event.0030
				save_scope_as = puppet_or_actor
				scope:old_elder = {
					add_opinion = {
						target = scope:new_disciple
						modifier = refused_to_be_disciple_opinion
						opinion = -50
					}
					save_scope_as = recipient #for loc in tgp_interaction_event.0030
					trigger_event = tgp_interaction_event.0030
				}
			}
			set_relation_elder = scope:new_elder
		}
		scope:new_elder = {
			add_merit_if_relevant_effect = {
				MERIT = $MERIT$
			}
		}
	}
}

As written in game/common/scripted_effects/10_dlc_tgp_scripted_effects.txt.