Black magic

Black Magic essentially breaks any of the Laws of Magic. It usually involves the cost of life and or free will.

Description
Anastasia Luccio, Captain of the Wardens: "For gaining control over others, for gathering great power to oneself, there's no greater tool than black magic."

To make magic work, you have got to believe in it, want it, or it just fizzles. That means that to kill someone with magic they must genuinely want it.

Fire could be a purifying force in its non-physical aspect—fire could consume and destroy dark magic if used with that intent. It was the weapon of choice for most Wardens.

In the Series
In Turn Coat, Harry and the Wardens fought the Grey Men on Demonreach with fire spells.