No one used fiendfyre anymore. It was Dark, volatile, and as likely to consume its creator as his enemies.
Yet some families preserved the knowledge, passing the incantation quietly down from generation to generation. Others discovered its secrets in a tome or carved on an ancient wall or woven into a tapestry.
No one used fiendfyre anymore. It was mysterious, unknown, and too powerful to control.
One would have to be both a powerful wizard and a complete idiot to unleash it in an enclosed space with nowhere to flee to. Fortunately for said idiot, the enclosed space in which he cast the spell also contained generations of hidden, broken, experimental, and forbidden magics of its own. Anyone else would have never cast the spell. Anywhere else, and he would have been incinerated and that would be the end.
No one used fiendfyre anymore. It destroyed nearly any substance, burning through stone and metal with impunity, but it also interacted with certain spells in unpredictable ways.
Certain shields could send it caroming off in random directions at speed - consuming the shield in an instant and obliterating anyone it touched. Certain attack spells would collide with the fiendfyre and change its colour or composition, or the size and intensity of its flames. And there remained countless magical artifacts whose innate abilities may react in any number of ways.
One such unknown was the Diadem of Ravenclaw. Another, a vanishing cabinet. Or the signet ring of a certain once-noble house, now subjugated to another.
No one used fiendfyre anymore.
But there was always an exception to any rule.