Finally smashed a writers block a few days ago. This popped into my head whilst I was trying to make progress on my other works. It is has not be Beta'd yet. This is just a oneshot for now. Don't expect any follows ups unless I hit another block or the fancy strikes me. I will not be focusing on this at all until I finish up a few of my current in progress stories.
Every Dark Lord has one particular spell they are remembered for; Voldemort and his Killing Curse, Grindelwald and his mastery over lightning, just to name the most recent memorable Dark Lords. It's perhaps a quirk of magic not just related to being a Dark lord, everyone has a spell that comes more easily to them than others. For Dark Lord Harry Potter that spell is Fiendfyre. The same indomitable will that allowed him to resist Voldemort's Imperius Curse denotes an unrivalled mastery over the cursed, magical fire.
Some claim that magic has its own form of sentience, and that is true. The large majority of witches and wizards never call on enough power for this to be readily apparent, but those who command power that boggles the mind, this fact is clear as the highest quality crystal. Only those with the power to sling Killing Curses around like candy, or form a corporeal Patronus that can drive away a hundred dementors would ever notice.
It is curious, that after such prolonged and consistent use, the sentient, cursed fire, Fiendfyre, no longer struggles actively against Potter's will. It bides it's time waiting for him to slip, but knows that under normal circumstances it will never overcome him. So it obeys his commands and incinerates those who oppose him to ash. Nobody believed he had the resolve to follow through on his path. How naive. Their attitudes changed quickly after he burnt down Gringotts, and everything within it, to the bedrock. In one fell swoop Potter crippled wizarding Britain. The economy suffered a critical existence failure, because nobody kept any significant source of money outside of Gringotts. Afterall, what kind of crazy maniac would willingly destroy the that which he was trying to conquer. All the money was gone, slagged along with everything else in the vaults beneath the once mighty bank. For those outside the know, it came out of the blue on a Monday morning. Potter walked up to bank and stood before the stone steps and stared at the white marble expression on his face hardened after some time. He held out his hands, and breathed life into a small spark of cursed fire that he had called into being. Fueled by magic that nobody alive in Britain could match, it quickly consumed the bank. Potter stood there watching the flames. He kept a tight hold on them, avoiding collateral damage. When his task was finished he turned and apparated away.