Among all the magical flames, there is only one that can be called unquenchable: the Fiendfyre curse.
This is the most powerful cursed flame in the wizarding world. It can burn through anything; there is nothing it cannot burn, not even water, the very nemesis of fire.
What's more, Fiendfyre grows more powerful the longer it burns.
This is a classic example of Dark Arts.
In the Ministry of Magic's records, it is listed as the most dangerous curse after the three Unforgivable Curses.
This doesn't mean that Fiendfyre is less powerful than the Imperius, Cruciatus, or Killing Curses. It's just that Fiendfyre's destructive power is a direct, head-on type of attack. It's not as unpredictable as the Imperius Curse, doesn't torture the body like the Cruciatus Curse, and doesn't have the instant-death property of the Killing Curse.
In terms of learning complexity, the Fiendfyre curse is far less complicated than Occlumency or the Patronus Charm. The difficulty of simply summoning Fiendfyre is not that high, and Dudley himself can do it.
The two old wizards, Pipilu and Luxixi, also knew the Fiendfyre curse, but they weren't good at controlling it.
And the most difficult part of the Fiendfyre curse is control.
Knowing how to control the path of the Fiendfyre is the mark of a powerful Dark wizard.
Simply summoning Fiendfyre without control will very likely lead to you being harmed by the curse yourself.
This is a curse that doesn't distinguish between friend and foe; it won't go easy on you just because you're the caster.
If you're not careful, the first person to be burned to death will be the wizard who summoned it.
There are countless examples of this in the wizarding world; it's a very common occurrence.
You should always go to an expert for expert matters. And who knows more about the Dark Arts than a Death Eater?
Of course, that's not exactly right.
In Dudley's mind, there was one person who could truly play with the Fiendfyre curse, a true master of the curse who was still alive.
Unfortunately, Dudley couldn't guarantee that this person would be willing to teach him.
So, the next best option was the 'Moody' he was currently with.
"An unquenchable cursed flame? Excellent, kid. You've come to the right person."
Just as Dudley had expected, Moody agreed very easily.
"I've fought against Dark wizards who use the Fiendfyre curse many times, so I've got some experience with it."
"Normally, I shouldn't teach you a dangerous spell like this, but since you've become a Hogwarts champion, you might have to face this curse in the near future..."
"So, get ready for some intense training. I can teach you on the first night of every weekend."
Moody took a sip from the flask of cloudy liquid on his belt, and a unique, medicinal scent wafted up to Dudley's nose.
Ah, that's the smell of Polyjuice Potion.
Moody, who was leaning on his walking stick, hadn't gone far when he suddenly turned back. "Oh, and when you come, bring Harry with you. As a Hogwarts champion, I think he should also learn some skills to deal with Dark wizards."
That last sentence was the most important part.
On Saturday, after their usual morning exercises, Dudley and Harry arrived early at Moody's professor's quarters.
Compared to Dumbledore's office, Moody's quarters were much smaller. The room was very simple, unlike Lockhart, who filled his room with portraits of himself, or Quirrell, who grew a pile of garlic in his quarters, or Lupin, who kept dangerous Dark creatures in his room.
Besides a few clothes hanging up, there was only a cauldron, a teapot, a few cups, and other miscellaneous things.
But the most striking thing was the large iron trunk by the bed.
It was secured with a large lock, looking very mysterious. He had no idea what was inside.
"Excellent. It looks like you both take this training seriously."
As soon as Dudley entered the room, he noticed his eyes lingered on the large trunk. Moody first shouted to get the boys' attention, then took a swig from his flask and motioned for them to come closer. "What are you waiting for? Come over here."
Seeing that the two students were a head taller than him, Moody discreetly took a few steps back so he didn't have to look up when he spoke to them. Then, he continued.
"I imagine you've both heard the incantation for the Fiendfyre curse before you came here, but I'll repeat it again..."
"Fiendfyre! This is the curse's incantation."
"Remember it, and now cast it for me."
Moody took another swig of his 'water.' "One at a time. First... Harry, you go first."
"Cast it? Here?"
Harry was clearly taken aback when he heard Moody's words.
This was Fiendfyre, not something harmless like the Aguamenti Charm. If they cast it here, wouldn't they burn down the castle?
Noticing Harry's confusion, Moody spoke, his voice full of a hint of disdain. "Don't worry. With your abilities, you'll only be able to summon a small handful of flames at most."
He held out his little finger and pinched it at the location of the nail bed with his thumb.
Although Fiendfyre was said to burn everything, in reality, the air couldn't be its kindling, meaning it couldn't burn through the air.
"I don't even need to use my wand; I can just breathe on it and put it out."
You guys might be diligent, learning quickly, and talented, but you're just beginners at the Dark Arts. What's the worst that could happen?
Moody was confident. Dark Arts were his specialty.
Just like Moody said in his first Defence Against the Dark Arts class, even if the students in his class cast the Killing Curse on him, the most he'd get was a nosebleed.
He was very confident in himself, and in Dudley and Harry.
Could a student really be stronger than a professor?
"But..."
Dudley remained silent, but Harry tried to explain something. Unfortunately, he was cut off by Moody as soon as he opened his mouth.
"No 'buts.' I said cast it right here."
Seeing Moody's attitude was so firm, Harry could only say helplessly.
"All right."
He pulled out his wand and pointed the tip at the ceiling, which was the area with the least amount of clutter.
"Fiendfyre!"
Harry said the incantation for the Fiendfyre curse. As soon as the words left his lips, a fist-sized, deep-red flame shot out from the tip of his wand. The flame leaped three feet high, hitting the ceiling and instantly setting it on fire.
The Fiendfyre curse burns everything. Even the least flammable objects can become kindling. The moment it touched the ceiling, it spread rapidly, and in a moment, it was burning its way along the ceiling toward the desks near the wall, looking like it would burn the entire room down.
The fire spread too fast.
Moody, who was originally planning to give Harry some proper guidance, was stunned. He quickly pulled out his wand and pointed it at the spreading Fiendfyre.
"Finite Incantatem!"
Unfortunately, this spell could only slow the spread of Fiendfyre; it couldn't stop the curse's threat.
But slowing it down was enough. Moody cast another spell.
"Fiendfyre Extinguisher!"