How did Crabbe manage to use such a powerful spell?

Crabbe appears to be a pretty fair wizard, at least when it comes to using dark magic. He was able to cast an Avada Kedavra at Ron a few seconds earlier with no evident difficulty.

Crabbe wheeled round and screamed, ‘Avada Kedavra!’ again. Ron leapt out of sight to avoid the jet of green light. The wandless Malfoy cowered behind a three-legged wardrobe as Hermione charged towards them, hitting Goyle with a Stunning Spell as she came.

_{^{Harry Potter and the Deathly Hallows}}

Latterly Harry surmises that Crabbe must have learned how to (inexpertly) cast the Fiendfyre curse from the Carrows.

‘Fiendfyre – cursed fire – it’s one of the substances that destroy Horcruxes, but I would never, ever have dared use it, it’s so dangerous. How did Crabbe know how to –?’

‘Must’ve learned from the Carrows,’ said Harry grimly.

‘Shame he wasn’t concentrating when they mentioned how to stop it, really,’ said Ron, whose hair like Hermione’s was singed, and whose face was blackened. ‘If he hadn’t tried to kill us all, I’d be quite sorry he was dead.’

The Carrows were, of course, said to be teaching the Dark Arts to their students. Crabbe and Goyle were top of the class.

‘Amycus, the bloke, he teaches what used to be Defence Against the Dark Arts, except now it’s just the Dark Arts. We’re supposed to practise the Cruciatus Curse on people who’ve earned detentions –’

‘What?’

Harry, Ron and Hermione’s united voices echoed up and down the passage.

‘Yeah,’ said Neville. ‘That’s how I got this one,’ he pointed at a particularly deep gash in his cheek, ‘I refused to do it. Some people are into it, though; Crabbe and Goyle love it. First time they’ve ever been top in anything, I expect.