rite_yearly_hor_divergence_drift_effect

rite_yearly_hor_divergence_drift_effect = {
	if = {
		limit = {
			this != faith.main_rite
			# We leave rites that are already heretical alone, since heresy already covers that range
			"divergence(faith.main_rite)" < faith.heresy_threshold
			head_of_rite ?= {
				is_available_ai_adult = yes
				NOT = { has_character_flag = hor_already_drifted_rite_flag }
			}
		}
		random = {
			chance = {
				value = 4
				# Low fervor: lots of room for reinterpretation
				if = {
					limit = { faith.fervor <= @hor_drift_low_fervor }
					add = 6
				}
				# Stable faith: rare drift
				if = {
					limit = { faith.fervor >= @hor_drift_high_fervor }
					add = -2
				}
				# Unfulfilled HoR's do changes more often
				if = {
					limit = {
						head_of_rite ?= {
							spiritual_fulfillment <= 0
						}
					}
					add = 4
				}
				if = {
					limit = { head_of_rite ?= { has_trait = heresiarch } }
					add = 25
				}
				min = 1
			}

			rite_yearly_hor_divergence_drift_apply_effect = yes
		}
	}
}

As written in game/common/scripted_effects/pam_effects.txt.