Muggles' Guide to Harry Potter/Magic/Fiendfyre

Overview
Fiendfyre is an all-consuming form of fire, that can be created magically.

Extended Description
Harry, Ron, and Hermione are looking for Ravenclaw's lost Diadem in the Room of Requirement when they are intercepted by Draco Malfoy, with Crabbe and Goyle. Knowing that whatever Harry is looking for could restore his family in the Dark Lord's good graces, Draco is intent on taking it from him. In the ensuing battle, Crabbe sets Fiendfyre, but evidently does not know how to stop it, and perishes in the flames.

The updraft from the fire does toss the Diadem up in the air, and Harry, then riding a broomstick, is able to catch it while making his escape.

Analysis
It is noted that the flames make the shapes of a number of vicious magical creatures, presumably this is part of the reason for the name of the spell. We don't see the spell being cast; and in fact we have no way of knowing why Crabbe would have used this spell anyway as its purpose seems purely destructive.

It should be noted that a Horcrux, as we learned in the early part of this book, can only be destroyed by more than ordinarily magical means. For instance, Basilisk venom, for which the only known antidote is Phoenix tears, is not something that can be cured by ordinary means. Basilisk venom has been used to destroy all of the Horcruxes found to date; the Diadem was destroyed by simply bouncing around in the flames of Fiendfyre for a short while. This is a clear indication that Fiendfyre is not an ordinary magical form of destruction, but is unusually powerful in its effects.

Questions

 * 1) The characters seem to imply that there would be a method to stop Fiendfyre, though neither the Trio nor Crabbe has learnt it. Hermione, on account of its danger, never even considers it as a method against Horcruxes, at a time when they know about no other thing in their reach that will do the trick. Precisely how dangerous, then, is Fiendfyre?