This herb is burnt, and the smoke inhaled, by a type of fairy called Rasta-fairy-uns. It causes them to become lighter, and allows them to fly.
Most intelligent creatures are far bigger and heavier than fairies, and so the smoke only makes them slightly lighter.
Scholars who have tested the herb say that it has various other effects, which they summarise as "far out". They intend to put out a more detailed warning, but haven't got around to it yet.
As a result of these warnings, Fairy's Fire is banned in many places. However it is cultivated and used openly in the Rasta-Fairy-Uns own country, the Nether Nether Lands.