It's called "Monstra te verum sui" (Show your true self) and the spell is used to find out the creature you're interacting with is a shapeshifter, or the original. You need to make the other creature look into the mirror (small, portable mirror is recommended), say the words, and then throw the mirror at the creature's feet. If it's the original, the spell will just create teal smoke. But if it's a shapeshifter, the spell will force it to change to its true form before the physical form starts melting.
Now here's the problem: Anti-Biotic, a shapeshifter, doesn't have a "default form". So she will experience agonizing pain and suffering until help arrives to neutralise the spell.
