"Like it hot, scum?"
-- Crabbe, while running past Harry trying to get away from the fiendfyre he just cast. (DH31)
Vincent Crabbe casts the Fiendfyre spell in the Room of Requirement. The fire gets out of his control, killing him and destroying Ravenclaw’s diadem, a Horcrux. Crabbe learned how to cast Fiendfyre from Amycus Carrow during his seventh year, but he did not learn how to control or stop it.
Everything that is being hidden in the Room of Requirement is destroyed (Pm).
Gryffindor is the Hogwarts house aligned with the element of fire (Syltherin is aligned with water.) (TLC). Crabbe's inability of keep control of the fire - and his finally being consumed by it - foreshadows the overall outcome of the larger conflict that is happening: the largely Gryffindor protagonists triumphing over the largely Slytherin antagonists. -BB
Fiendfyre flames can take the shapes of different creatures. In the film Harry Potter and the Deathly Hallows: Part 2,the Fiendfyre takes the form of each of the mascots of the four Hogwarts houses. -BB
Crabbe's last moments give some insight into his character. For the first time, Crabbe has turned on Draco (it appears that Crabbe sees no need to be Draco's henchman now that the Malfoys have fallen out of favor). Crabbe never spoke much, and Harry is surprised by how soft this big, brutish wizard's voice is.