For seven days, Cassius read until his eyes ached.
Grindelwald's handwritten parchments were dense—histories of fallen empires, treatises on war, runic commentaries tracing the origins of magical law.
Cassius absorbed what he could, though frustration grew with each night.
He wanted fire in his veins, not ink on his fingers.
On the eighth morning, however, things changed.
Grindelwald summoned him to the center of the chamber, staff in hand.
His mismatched eyes glittered like frost and flame.
"Theory has its place," the old man said, voice carrying the weight of command. "But it is hollow without experience. Today, boy, we leave parchment behind. Today, we test the marrow of your understanding."
Cassius straightened.
At last.
Yet instead of raising a wand, Grindelwald sat, motioning for him to do the same.
"You are impatient," Grindelwald said, settling heavily into his chair. "And so was I, once. Perhaps you will understand me better if I tell you how impatience nearly destroyed everything."
Cassius hesitated, then sat opposite him. "You speak of your revolution?"
"No." Grindelwald's voice lowered, gravel rasping in the quiet. "I speak of Albus. And the sister he lost."
Cassius blinked.
This was a story he knew, or at least partly knew from the portion written by Rita Skeeter in the future.
"Tell me,"
Cassius urged.
Grindelwald's gaze turned inward, seeing not the stone chamber but memories long buried.
"When I was young, I met Albus at Godric's Hollow. We were brilliant, ambitious, insufferable. He was caged by duty, bound to his family after imprisonment of his father and death of his mother. I was untethered, blazing with visions. Together, we dreamed of a new order—wizards rising to their rightful place, leading the world rather than skulking in shadows."
His lips curled faintly, half-smile, half-sneer.
"It was intoxicating. For a time, I believed nothing could stand in our way. Not even fate itself. Dreaming about becoming gods among men, wielding their weapons with wild abandon."
Cassius leaned forward, hungry for every word, knowing the 'gods' weapons were refering to the hallows.
"But something did."
"Yes. Aberforth." Grindelwald's voice sharpened like glass. "Albus' younger brother. Stubborn, provincial. He hated me from the first, and with reason—he saw me for what I was. Not just a friend to Albus, but a spark dragging him toward a fire he could not control. Aberforth demanded Albus abandon our plans, that he remain at home, caring for his sister Ariana."
Grindelwald paused, his mismatched eyes narrowing.
"Do you know of her?"
Cassius shook his head, a lie yes, but how else could he explain knowing who the squib of a girl was at this time.
"She was a frail girl, quiet, withdrawn. But there was a darkness in her—a curse born from the violence she endured as a child at the hands of muggle boys. Traumatized, her magic turned inward, unstable. She became an Obscurial-in-waiting."
Cassius stiffened.
He had read enough to know what that meant: an Obscurus, a parasitic force of destruction born from repressed magic, often killing its host before adulthood, but it was a mass of pure magical power, generally vastly most powerful than any wizard or witch who regulate their power.
Grindelwald nodded at his silence.
"Yes. She was dangerous without meaning to be. A living powder-keg in muggle terms. Aberforth swore to protect her. Albus… tried to balance both her and me. And I—"
His smile turned cold.
"—I cared only for the Cause."
His hands tightened on his staff.
"The confrontation was inevitable. Aberforth challenged me, shouting that I had ensnared his brother, that my visions meant nothing while Ariana needed care. He accused me of using Albus, of stealing him away. My pride… my pride could not bear it. Words turned to curses."
Cassius' pulse quickened.
He could almost see it, sparks flying in that little basement, the clash of ideals not on parchment but in fire.
"Albus tried to intervene. But I—"
Grindelwald's face hardened.
"—I drew first blood. My curse was meant to end Aberforth's meddling. To break him. Maybe even end him from getting in my way."
He exhaled, long and slow.
"Albus entered the duel. Three wands clashing in fury. I remember the light, the sound—walls cracking, air screaming with magic. And in the corner, Ariana, cowering, her magic thrashing within her like a storm. None of us saw her until it was too late."
Cassius swallowed.
"She… died?"
Grindelwald's gaze turned bleak, as though a century of regret condensed in that moment.
"Yes. A curse rebounded—whose, I do not know. Perhaps mine. Perhaps Albus'. He believed it was his. I believed it was mine. In the end, does it matter? Ariana fell. The girl became a body on the floor, and the three of us—the greatest wizards of our age—were left broken children in her shadow."
For a moment, silence filled the chamber, thick and heavy.
Cassius' voice was barely a whisper.
"And Albus?"
"Destroyed." Grindelwald's tone was sharp now, a blade against memory. "His brilliance turned to ashes. He would not follow me. He would not lead the world. He turned inward, swearing to bury ambition beneath penance. I left that night, driven by fury, shame, and conviction. If Albus would not stand beside me, I would stand alone."
Cassius knew, dumbledore retreated from the world for close to 30 years after that night, fanfiction would claim that his time spent was researching obscurials looking for any chance his sister could exist in some other way, since Obscurials were in essence pure magic without a body, similar in a way to Ghosts.
Then his obsession turned later on to hunting for the hallow he previously cared least about, the ressurection stone, a tool meant to lead one to their death, but to him a way to atone for his sins.
The old man's mismatched eyes locked onto Cassius.
"Do you understand, boy? The greatest revolution of our age was born in the death of a child. Every empire carries a corpse at its foundation. The only question is whether you let it haunt you… or drive you."
Cassius' breath came shallow.
The story was more than tragedy—it was warning.
A glimpse of what ambition, unchecked, could cost.
But also… a lesson.
"You regret it," Cassius said slowly.
"Yes." Grindelwald's voice was iron. "But I would not undo it. Regret is not the same as repentance. Ariana's death revealed to me a truth I had ignored: power demands sacrifice. The only choice is whether the sacrifice is chosen… or accidental."
Cassius felt the words like brands in his chest.
His mind flickered to his own family, his twin, the shadow of prophecy coming from his mentors mouth.
How many corpses would his path demand?
Grindelwald rose, leaning heavily on his staff.
"Enough stories. You came for lessons. Let this be your first true one: Never fight for power without knowing who will pay the price. And never let the price be paid by accident. Choose your sacrifices."
With a flick of his staff, Grindelwald transfigured a portion of the room into what looked like a yoga studio.
Though one with a mat in the center but ritualistic looking obelisks on the sides.
"Get in the center, today you face your first trial."
Grindelwald stated almost giddily.
"Many assume Albus, I, and even your Lord Voldemort were naturally gifted, destined to become powerful wizards at the peak of what magic can be, but they would be wrong, instead what we all engaged in was something that most magical borns are told never to do."
Cassius seated on the cushion with his legs crossed tilted his head unknown where his mentor was going with this.
"You've probably never heard the term, Magical Riot?"
"No."
"Alright, fitting, this experience is often overlooked by our kind afterall, a magical riot is like an awakening of ones magical talent, in muggle borns its something that happens spontaneously with intense emotions, and generally only happens once before being prevented after getting warned by the various ministries to protect the secrecy statutes, while wizard family are less inclined to prevent them but also only celebrate the first before moving on."
Cassius nodded along, since this matched with his knowledge.
"But, what we did was train our magical riots, rather than only experiencing it once, we would induce them to occur as often as we could, letting our magic flow out like an intense wave, being able to affect our surroundings like pure unfiltered magic, no real control in the beginning, but so very few children can control their emotions. And while we could make object levetate it could not be called 'our' magic."
Grindelwald moved on to pull out a diagram of the human body.
"In wizards we have magical pathways, in which out magic flows, all coming from a core, a magical organ if you will that is the source of all our magic, just like cores found in magical creatues, only that for humans our pathways and core are in flux, a fluid like state until finally stabilizing as one enters puberty. But like physical training these pathways and core can be trained, and strengthen though its dangerous to do so. Should you riot to hard, you magic will explode like an obscurial damaging your core and pathways and rendering our inert like a squib."