The [Incomplete Rhongomyniad]? If I remember correctly, it deals true damage to an enemy without using mana but using your health as the means to cast it. However, that's about it. True damage doesn't mean you will be able to penetrate its invincibility. So even if I fire it off, it will be useless.
"Why do I have to use it? It is pretty useless against the boss who is under the effects of invincibility! Firing this off is useless and I would just waste my health in the process and I won't do any kind of damage at all!"
"Just fire it, master, trust me. I know what will happen. Since it is incomplete, the info given out too is incomplete. Just trust me on this one."