Forum:Number 96 Dark Mist ATK?

Is the atk boost of Number 96: Dark Mist a permanent boost since it doesn't specify for how the boost is? Or does the fact that it can only be activated once per battle constitute that it only lasts for that battle? Fabled Ashenveil does specify the duration as well as saying once per battle, but the wording is drastically different so it's unclear as to whether that would be applicable in regarding 96's effect.