Regulus never asked about the window directly. Not one word of it.
But the theory he needed could come from McGonagall all the same.
She wouldn't hand him an answer about the Decomposition Curse. Transfiguration theory, though, could tell him whether applying a shape to something stripped of its true form held together logically.
If it did, the rest was execution.
How long the window ran, how to complete a transfiguration inside it, how to keep those basic units from scattering, all that was his own problem.
She was quiet a while, taking the idea seriously. It had depth, whatever else could be said about it.
Strip a thing until it was no longer defined as anything determinate, then confer a form on it there. That wasn't ordinary Transfiguration anymore. It touched the border between existence and form.
She'd turned over questions like it herself across a long career of study.
But she had decades behind her, and the child in front of her was a second year who had followed the inclination to return all the way to that border on his own.
In her years of teaching, that kind of mind came along rarely.
"On the level of theory alone," she said at last, her voice gentler than before, "the reasoning holds. If a thing truly retained nothing of its own true form, there would be no direction left to return toward, and the inclination could not arise at all."
He listened, fixed on every word.
"There is a fundamental difficulty, however. Something stripped of every conviction about itself will not sit quietly in that state waiting for you. It is no longer anything, and such a state cannot sustain its own existence. Reaching that point, it either scatters in an instant without leaving a trace, or it collapses back on itself at speed and settles into some form again. The condition you're imagining, a thing that is nothing at all and can be shaped as you please, leaves you no comfortable stretch of time to work in. It is a seam with almost no width to it."
Thought crossed his face.
Every word of that landed somewhere solid in him.
Especially the collapse.
Under the Decomposition Curse he had only ever seen things scatter. Collapse back into some form, never.
Perhaps at a certain degree of decomposition there was another outcome he simply hadn't run into.
He set the thought aside and kept listening.
"There's a further layer," she went on, watching him. "Even supposing you did confer a new form inside that seam, what you have is no longer the original object. The original ceased to be the moment you stripped away its true form entirely. What you obtain is an altogether new existence, not an old thing permanently transfigured. That falls outside Transfiguration. Transfiguration deals with the form of what already exists. Erasing a thing completely and then establishing something new out of nothing is another matter."
She stopped there and went no deeper. She had drawn the border, and now she waited.
To her mind this was a gifted young wizard's theoretical flight along the edge of the discipline.
Deep, but theory in the end.
"That is as far as Transfiguration can take you," she said, her tone cooling a little. "Beyond that point you're asking questions for some other field to answer."
He dropped his eyes, thinking fast.
She hadn't given him an answer. He had the answer he wanted regardless.
That seam with almost no width was a precise portrait of the window.
In her mouth it was a crack too narrow to be worth attempting, a theoretical warning meant to send a student home. In his, it was a blueprint.
He could take a living thing apart at the level of existence and make it no longer itself. That step he could manage.
After the unmaking, though, the thing either scattered or collapsed back, and the window between no longer itself and not yet anything else was brutally short.
He'd never even witnessed the collapse. With the Decomposition Curse he'd seen only dispersal, everything gone in a breath.
Perhaps he had done it too thoroughly every time, leaving no room for a collapse. Worth verifying later, with someone to test on.
He had guessed the window existed. He had never once considered doing anything with it.
What followed the breakdown he'd always treated as the tail end of the deconstruction. Let it scatter and be done.
By what she'd just described, the seam wasn't only an ending. Something could be done inside it. A form could be given.
Apply a transfiguration in that crack, mold the units before they dispersed into a new shape, and you'd have a form with no original behind it. No old true form to suppress. Nothing to return to, ever.
The new built on the ground where the old had ceased to exist. While no true form remained, make the new shape the only fact there was.
An alternate kind of Permanent Transfiguration. Beyond the bounds of Transfiguration entirely, if she was right.
Decomposition and shaping. The two could be joined after all.
Something in it.
McGonagall watched him, quietly pleased.
The office went still for a moment.
She lifted the teacup from the corner of the desk, drank, and looked at him over the rim.
The younger Black could be stubborn in his thinking. Spatial Transfiguration had been the same, magic hurled against the elasticity of space out of sheer refusal to consider anything else.
Too reliant on reason. Not enough magic in him yet.
But the mind was sound. He dug toward essentials, understood at a word, and carried the reasoning forward himself.
Whatever she taught him took root. Not like most students, where the words went out and nothing came back.
A student like that was a relief to teach.
She set the cup down, the base clicking against the saucer.
Regulus gathered up what she'd said.
Two roads from her.
Suppression, which left the old true form untouched and simply held it flat with magic.
Conversion, which rewrote the old true form so the object accepted the new shape, the contradiction dissolved, the inclination left with no root, nothing to maintain.
He'd found a third.
The Decomposition Curse. Wipe the old true form out altogether. Delete it.
Tear that whole structure out by the root, then, in the window before dispersal and before re-formation, give it a new shape.
That road she hadn't named.
Suppression and conversion lived inside the framework of Transfiguration. Reshaping after erasure did not.
The magical world was wide. Some wizard somewhere might do something like it, and he had no idea what she'd seen in her life.
But sitting in this office, she had cut off at the border of the discipline and placed reshaping-after-erasure outside it.
She hadn't said it didn't exist. She hadn't said she didn't know. Only that it wasn't Transfiguration.
Suppression. Erasure. Conversion.
The three lined up in his head.
Suppression meant force holding the old true form down. Erasure meant scrubbing it out. Conversion meant letting the old true form become the new one itself.
The first two were brute solutions. Pin it, or delete it.
The third worked differently. It destroyed nothing. It only let the thing turn into something else.
No opposition, only guidance. No deletion, only conversion.
A question he'd shelved a long time ago brushed against those three roads.
Not suppression, not deletion, but guiding a thing to convert on its own. If Transfiguration could handle a true form that way, what about other things?
Light and dark.
Ever since the empty house in Greyback's mental architecture, he'd been asking whether isolation was a dead end.
Shutting the dark away in the Containment Room, never touching it, never letting it near the light. Wasn't that another form of suppression?
The same as holding an old true form down so it couldn't surface?
Held down, it doesn't act. Held down doesn't mean gone.
The old true form is always there, waiting for the instant your grip loosens to spring back.
The dark side was no different. Isolated, it caused no trouble. Isolated didn't mean it had vanished.
It sat in the Containment Room, waiting for the day the walls failed and it could pour out.
Suppression and isolation ran on the same logic underneath. Don't touch it, just press.
Erasure, then?
Delete the old true form whole. Clean. Absolute. No aftermath.
But deletion was annihilation, not conversion.
Wipe out the dark and let the light stand alone. A road, certainly. Not one he'd take. He wanted both.
And as she'd said, let the old true form accept the new shape and no outside force is needed anymore, because the contradiction is gone at the root.
Could light and dark work the same way?
Not the light suppressing the dark. Not the light destroying it. The dark converting itself.
Conversion might be right. How to convert, and into what, he had no idea.
Coexistence was settled as a direction. That was a framework. The path remained dark.
Greyback's empty house had told him only that isolation fails. Voldemort's fission had told him only what tearing a soul apart costs. His own fusion had told him only that the bright side could be lit brighter.
What to actually do with the dark side, he still had nothing.
Maybe once the darkness gathered, once another piece of the light-and-dark puzzle turned up, the shape of the question would surface.
The thought had barely formed before he pushed it down. Wrong moment for it.
Nothing could be done now regardless. The seam was too short. He couldn't catch it yet.
Testing it meant a living sample and some means of controlling the seam, and he had neither.
Joining decomposition to transfiguration, seizing the operational window, every step demanded extreme precision of mind and magic.
Once Saiph was lit, this could be the first thing he verified.
He drew his thoughts back in.
"Thank you, Professor." He raised his head, sincere. "I got a great deal out of today."
She nodded and waved a hand.
"Anything in Transfiguration, ask," she said lightly. "What you can't work out, think about longer. What you still can't reach, come back to me."
Then the severity returned. "Your essay is due before Friday."
He paused, then inclined his head. "Yes, Professor."
A knock sounded at the office door.
She glanced toward it and raised her voice. "Come in."
The door swung open. Sirius stood in the doorway.