The moment Erwin finished speaking, his amethyst wand snapped into position. A flash of light shot straight toward Grindelwald.
Grindelwald remained perfectly composed. With a casual flick of his wrist, he produced his wand and cast his own counter-curse. The two spells collided head-on in a shower of bright sparks.
Erwin didn't press the attack. He knew today wasn't about dueling—it was about teaching the students.
Grindelwald turned to the gathered class. "When attacked by a spell, you can intercept it with the same incantation. Just now, Erwin used the Disarming Charm, and I mirrored it. In a perfect match, the spells neutralize each other." He let that sink in. "However, there is a critical factor: magical power. If the gap between you and your opponent is too wide, the result changes entirely."
He nodded at Erwin. "Show them."
Erwin raised his wand again, casting another Expelliarmus.
Grindelwald mirrored him perfectly. But this time, the result was starkly different. Erwin's spell tore through Grindelwald's, overwhelming it completely. The remaining force struck Grindelwald's wrist, sending his wand skittering across the stone courtyard.
Grindelwald calmly retrieved it. "Just like that. If your magic is weaker, your spell is obliterated. It's a harsh, unforgiving law of dueling."
Hermione raised her hand. "Professor, how do we judge the strength of an opponent's spell before it strikes?"
Grindelwald's eyes gleamed with approval. "Miss Granger. I've heard the professors speak highly of you. You cut straight to the heart of the matter." He paused. "Unless you're a master of Legilimency, you cannot know exactly how much power an opponent is channeling until they cast. But there is a technique."
He gestured for Erwin to join him. "When two experienced duelists fight, they often match power instinctively. Today, I'll show you how to read intent."
Grindelwald approached Erwin and gently touched his wand tip to Erwin's fingers. "A wizard's intent is mirrored in their body. Through stance, tension, and the grip on their wand, a keen observer can estimate the spell's potency before it's even cast."
He stepped back. "This isn't a closely guarded secret—it's the first lesson every Auror learns at the Ministry. Survival begins with observation."
Pansy Parkinson raised her hand. "Professor, if everyone knows this, doesn't it make hiding your strength impossible?"
"Precisely," Grindelwald said, his voice taking on a grave tone. "That is why the truly powerful train to mask these tells. They conceal their intent until the moment of casting. I know you train through simulations, but those are sanitized. You must learn to read the reality of a battle with your eyes and your instincts. This is the foundation of survival—to recognize the threat before wands are even raised. Learn to read your enemy, and you gain the advantage before the first spell is cast."
The students nodded, their expressions serious. They were no longer naive first-years—they understood the weight of the lesson.
Grindelwald turned fully to Erwin. "Now, watch closely. See how true wizards duel."
The atmosphere shifted instantly. The playful instructor vanished, replaced by the legendary Dark Wizard of old. Erwin's expression hardened. He respected Grindelwald's power—perhaps even more than Dumbledore's. It was raw, ancient, and refined by decades of conflict.
The students scrambled back, clearing a wide circle on the grass.
Magic erupted from both men simultaneously. It wasn't just casting—it was a manifestation of pure will. Purple flames danced around Erwin, wreathing him in crackling energy. Around Grindelwald, the air shimmered with a deep, chilling blue radiance.
The sheer pressure of their magical reserves slammed against the castle like a physical wave.
In the Transfiguration classroom, Professor McGonagall paused mid-lesson. She felt the surge instantly. Walking to the window, she saw the confrontation in the courtyard. A small smile touched her lips. Grindelwald was certainly unconventional, but she was curious to see how her Head Prefect fared against a legend. She didn't stop the students who tiptoed to the windows—she watched alongside them.
From every classroom in the castle, heads appeared at windows. Professors halted their lectures, drawn by the sheer magnitude of the magical pressure.
High above, Dumbledore appeared, hovering in the air. He'd sensed the duel before he saw it. He watched with keen interest—a rare opportunity to witness his old rival in action, and to see just how far Erwin had truly come.
Down below, Grindelwald chuckled, his voice carrying over the crackling of magic. "It seems we have an audience, Erwin."
Erwin shrugged, his grip on his wand steady. "Your reputation precedes you, Grindelwald. They want to see the man who once challenged the world."
"And you?" Grindelwald asked, his blue eyes flashing with intensity. "You show no fear. Are you testing yourself against me, or are you merely demonstrating for the students?"
"I'm ensuring the students learn the difference between theory and reality," Erwin replied, a ghost of a smile on his lips. "Let's see what you've got."
Grindelwald laughed, the sound carrying across the grounds. "Then don't disappoint me."
With a sudden, explosive movement, they lunged.