10 Deadliest Spells In The Harry Potter Universe

What does it do? Conjures a massive spread of cursed fire that can seek out live targets, and cannot be extinguished by conventional means.

Fire, in general, is rather dangerous, but it's even more dangerous when said fire becomes almost sentient, and can hunt you down like one of the raptors from Jurassic Park.

And unfortunately for Harry, Ron and Hermione toward the end of Harry Potter and the Deathly Hallows Part 2, that's exactly the predicament they find themselves in - being chased through the maze-like Room of Requirement by an enormous, bird-shaped cloud of flames.

Fiendfyre is infused with Dark Magic, which makes it incredibly hard to control for inexperienced casters. In the aforementioned film, the spell is cast by Gregory Goyle, who displays this lack of experience when he loses control of the fire, attempts to evade it and falls to his death.

There is a charm that can subdue the flames, but it's very complex and less skilled wizards will likely have lost control of the fire before it can be of any use. Fiendfyre's saving grace is that it can be used to fully destroy Horcruxes, but that's probably of little comfort when you're being chased by an angry, self-aware flame-monster.