Fiendfyre

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

• One of the substances that can destroy Horcruxes, but Hermione considered it far too dangerous to ever use (DH31).
• Crabbe cast this in the Room of Requirement while it was in its Room of Hidden Things mode (DH31). He died in the ensuing magical inferno.