Did Voldemort use Fiendfyre against Dumbledore during their duel at the end of Order of the Phoenix in the Ministry of Magic? Here's the specific passage I'm wondering about:
A long thin flame flew from the tip [of Dumbledore's wand]; it wrapped itself around Voldemort, shield and all. For a moment, it seemed Dumbledore had won, but then the fiery rope became a serpent, which relinquished its hold on Voldemort at once and turned, hissing furiously, to face Dumbledore.
Voldemort vanished; the snake reared from the floor, ready to strike –
There was a burst of flame in midair above Dumbledore just as Voldemort reappeared, standing on the plinth in the middle of the pool where so recently the five statues had stood.
‘Look out!’ Harry yelled.
But even as he shouted, another jet of green light flew at Dumbledore from Voldemort’s wand and the snake struck –
Order of the Phoenix - pages 718-719 - chapter 36, The Only One He Ever Feared
Crabbe set the Room of Requirement on fire using Fiendfyre and ended up dying as a result because he didn't know how to control the fire. Two wizards as powerful as Dumbledore and Voldemort would be able to control Fiendfyre well enough to use it in a duel, I believe. However, we only see a snake form from whatever type of fire Dumbledore conjured, and not multiple creatures like how Fiendfyre presented in the Room of Requirement. But again, that could go back to Crabbe not knowing how to control it. Is the fire used in Dumbledore and Voldemort's Ministry of Magic duel Fiendfyre, or is it some other kind of magical fire?