The aura only consumes a charge and applies cancelEvent, damageAdd or damageMultiplier if the onAttackSkill executed successfully: its chance roll succeeds, it is not on cooldown and all of its conditions pass. If the skill fails, the damage event is left untouched and no charge is consumed. If no onAttackSkill is configured, the execution always counts as successful.
Additionally, cancelEvent takes precedence over the damage-modifying attributes: if cancelEvent is true, the hit is cancelled and damageAdd and damageMultiplier are ignored.