Cursed Fire which is very dangerous and made from Dark Magic. Vincent Crabbe set it off in the room of requirement in Deathly Hallows and died because he couldn't get out or put it out.
Vincent Crabbe died because he cast FirendFyre in the room of requirements to try to kill Harry Potter. The spell quickly burned everything and Harry, Hermione, and Ron were able to escape by using brooms. They tried to save Malfoy, Goyle, and Crabbe, but Crabbe slipped off the broom and fell into the fire.
Crabbe is killed by his own FirendFyre in the room of requirement on May 2, 1998. In the seventh book, when Harry, Ron, and Hermione are trying to find Horcruxes at Hogwarts, they go into the Room of Requirement because Harry remembered seeing the diadem of Ravenclaw(a Horcrux) in there when he hid his Potions book in his sixth year (The Half-Blood Prince). They split up and are looking for it among the piles of junk when they are ambushed by Malfoy, Crabbe, and Goyle. Crabbe performs a spell that releases Fiendfyre (cursed fire, one of the substances that destroys Horcruxes) and loses control of it because he doesn't know how to stop it. Anyway, he eventually ends up consumed by his own curse and that's how he dies. Malfoy and Goyle are saved.