Fiendfyre is invoked corporeal fire with high destructive power. It is usually given the shape of an animal, real or imaginary. Fiendfyre can move and change shape at the caster's will but requires a strong will to be controlled; otherwise it can turn upon the caster and consume them.
Fiendfyre can be produced with the incantation az-reth repeated 6 times while tracing the Sigil of Cold Earth with one's wand. The spell requires the permanent sacrifice of one drop of blood, i.e., according to Tom Riddle, after casting the spell, the caster's body would forever "be lighter by that drop of blood". According to Bellatrix Black, az-reth is the name of a creature that is being summoned by the spell.
The spell can destroy very old and strong magical artifacts. It can melt the walls of Hogwarts and kill a phoenix. However, the flame simply vanishes when sent against the Mirror of Perfect Reflection. Fiendfyre has no effect on dementors.
Fiendfyre can be sucked into a vacuum. This is implemented as part of the Tower's defence mechanisms and successfully disables Bellatrix Black's Fiendfyre during her attack on the Tower on April 30th, 1999.