The fire in the Room of Requirement set by Vincent Crabbe is a cursed fire known as the Fiendfyre. It is a very dangerous usage of Dark Magic, and it burns almost everything that comes in its way. Luckily for the trio and Draco and Goyle, they were saved because of quickly using broomsticks to escape the Room of Requirement. Unfortunately for Crabbe, his very creation led to his demise as he burned in the Fiendfyre.