"Fiendfyre-cursed fire — it’s one of the substances that destroy Horcruxes, but I would never, ever have dared use it, it’s so dangerous."
-- Hermione Granger (DH31)
Effect: Creates a cursed fire, made up of flames of abnormal size and heat that can crumble fairly substantial objects to soot at a mere touch. Left burning long enough, the fire will take the shapes of gigantic fiery beasts (including serpents, chimaeras, dragons, and birds of prey) which will pursue any target humans.
References from the canon
"fiend" + "fyre", from Old English "fyr", fire