Dust of Osiris is the all-purpose Mystic Code that Rani develops for you to use in your battle against Passionlip.
You use this in order to avoid instant death from Lip’s cube compression attack. This Mystic Code is a raging sand storm that blocks anything that would try to affect/change your status.
To find out why it has the name “Osiris” please refer to the last boss of the fighting game Melty Blood Actress Again. It blocks Green Archer’s poison, and even Karna’s meteorite attack.
What was that soothing aromatic tree? Well in any case Rani-san alone is good enough, right?