As Isaac's Summoning Charm took effect, the Diadem of Ravenclaw shot toward him.
When the Diadem was halfway there, Professor Vinda stepped out from behind Isaac.
She pointed her wand straight at the airborne Diadem, and flames poured from its tip.
These were no ordinary flames.
Standing beside Professor Vinda, Isaac could sense that better than anyone.
The flames were alive. Aware.
Fiendfyre.
Fiendfyre was no ordinary fire. It possessed a degree of awareness and perception, and actively sought out and pursued living things.
The flames also constantly changed shape, becoming enormous fire dragons, Chimeras, Fire Serpents, birds of prey, and other beasts that devoured their quarry with fangs and claws.
Most importantly, once cast, the Fiendfyre Curse kept consuming anything flammable and growing stronger. Its power increased exponentially.
In the original timeline, Malfoy's lackey Crabbe had used the Fiendfyre Curse in the Room of Requirement.
But the guy couldn't control his Fiendfyre at all. Ordinary dark wizards generally avoided the Fiendfyre Curse for that very reason.
They couldn't control the thing themselves.
But Professor Vinda was no ordinary dark wizard.
She had been Grindelwald's right hand, and Grindelwald was a master of Dark Magic who could make Fiendfyre dance to his tune.
When Professor Vinda unleashed Fiendfyre, it didn't immediately begin devouring nearby combustibles to fuel its growth.
Instead, it took on the shape of a beast in midair.
The surging Fiendfyre became a bird of prey, swooping toward the falling Diadem to rake and peck at it.
Just as the Fiendfyre bird was about to touch the Diadem of Ravenclaw, a vicious shriek rang in Isaac's and Professor Vinda's ears.
Then a mass of dark, tar-like substance gushed from the Diadem of Ravenclaw like blood.
A thick black mist followed.
Isaac could see it clearly: the mist seemed to be trying to take human form. Unfortunately for it, it had emerged too late.
The shriek from Voldemort's Soul Fragment did not faze Professor Vinda.
Please. She had once worked for Grindelwald. When Grindelwald was sweeping across Europe, little Tom might not even have started school yet.
Under Professor Vinda's control, the Fiendfyre bird plunged straight into the black mist. Voldemort's Soul Fragment screamed again as the flames began fiercely burning the Diadem of Ravenclaw.
Seconds later, the screaming stopped abruptly, and the Diadem, which had been floating on its own, fell to the floor.
Once the Diadem was burned, Professor Vinda had no time to inspect the remains. She had to deal with the Fiendfyre she had created.
That was the trouble with the Fiendfyre Curse. Its danger lay not so much in the spell's power.
It lay in the fact that the flames had a will of their own and could not be controlled. Worse, they could grow without limit by igniting anything flammable.
Grindelwald might have been able to control Fiendfyre at will, but Professor Vinda could only exert limited influence over it.
As she had just done: she had kept the Fiendfyre from devouring combustibles and directed it to consume the life force belonging to Voldemort inside the Diadem of Ravenclaw first.
Put simply, once Fiendfyre appeared, it had to devour the life around it. That was its nature.
Professor Vinda could make it bypass life forces A and B to devour C first: Voldemort.
But once C was gone, the Fiendfyre would turn to A and B: Professor Vinda and Isaac.
By then, it would be harder and harder to control. The Fiendfyre's own will would keep fighting Professor Vinda for command of the flames.
Fortunately, now that she had destroyed Voldemort's Horcrux, Professor Vinda no longer needed to force the Fiendfyre to obey her.
She immediately stopped casting the Fiendfyre Curse. Freed from her restraint, the Fiendfyre instantly changed from a bird of prey into an enormous Fire Serpent.
Isaac couldn't shake the feeling that something was wrong with its head. Why was it so round? And why didn't it have a nose?
Isaac's thoughts moved fast, but Professor Vinda moved faster.
The moment she ended the Fiendfyre Curse, Professor Vinda pointed her wand down. For the first time, she spoke a formal incantation aloud.
"Finite Incantatem!"
Professor Vinda's wand sank easily into the floor of the Room of Requirement, and brilliant golden light sprang forth.
The light shot upward, forming a straight golden pillar from floor to ceiling.
The noseless Fire Serpent was driven skyward by the light and slammed hard into the ceiling.
The golden light blocked the sparks that scattered from the impact.
Finally, the light spread across the floor, forming a huge golden ring beneath Professor Vinda's feet. A barrier rose from it.
The ring's edges curved upward, gradually forming a transparent golden dome that sheltered Professor Vinda and Isaac.
The barrier's surface was covered in intricate alchemical symbols and ancient magical patterns that flickered as energy flowed through them.
Under Professor Vinda's steady direction, the barrier protecting them expanded until it finally extinguished the Fire Serpent overhead.
Only then did Isaac truly see how terrifying a powerful Wizard's magic could be in a real fight.
Dumbledore's fight against Voldemort's wandering soul didn't count.
Back then, Dumbledore had thrashed Quirrell like he was his son... No, like he was his grandson.
So in terms of spectacle, it hadn't come close to this.
Watching Professor Vinda clean up the Fiendfyre she had made showed Isaac what a powerful Wizard could do.
It also made him warier of the Fiendfyre Curse.
Isaac had originally been keen to learn it. Grindelwald's gas burner was practically the best advertisement the Fiendfyre Curse could ask for.
Anyone who could learn magic and had seen that spectacle would be desperate to try Grindelwald's Fiendfyre gas burner at least once.
But the thing was genuinely hard to control. One mistake, and it would turn on its caster.
Professor Vinda hadn't even let it freely devour the combustibles around them, yet the usually elegant professor was already panting.
If Isaac secretly practiced it at Hogwarts and accidentally set the castle or the Forbidden Forest ablaze, who else would end up in Azkaban?
More importantly, Isaac had always believed that a spell only truly belonged to a Wizard if he could control it completely.
Perhaps one day, when he was stronger, he could use Fiendfyre to show off whenever he liked.
For now, though, he would put his study of the Fiendfyre Curse on hold.
"Professor Vinda, are you all right?" Isaac asked softly. By then, Professor Vinda had caught her breath.
The two of them went over to where the Diadem of Ravenclaw had fallen.