Regulus knew better than to ask the wrong questions, better than to let slip knowledge he shouldn't have.
He adopted the tone of a student seeking guidance, threaded with curiosity and a touch of deference.
"Professor, a physical transformation of that degree... aside from prolonged exposure to Dark Magic or deliberate bodily modification, could there be other causes?"
Slughorn studied the boy. Something complicated moved behind his eyes. Regulus kept his expression steady, open, earnest, a student asking to learn.
A long silence passed before Slughorn finally spoke, falling back on the cadence of a lecturer.
"The causes of physical transformation, viewed through the lenses of potions theory and magical pathology..." His tone steadied, finding its footing on familiar ground. "There are several possibilities."
"The first: prolonged exposure to high-concentration Dark magical artifacts. Cursed vessels, for instance. Over months and years, they erode the body from the outside in, starting with the skin and hair, working deeper. There was a Hungarian dark wizard last century who crafted a suit of armour from human skin. Living skin. It could absorb a Killing Curse, shedding one layer per hit. But the cost was that the wearer's own skin slowly fused with it. After a few years, you couldn't remove it anymore. And the man beneath had stopped looking human."
Regulus listened, filing it away. Chronic external erosion, outside-in, measured in years.
An interesting line of thinking, too. One worth remembering.
The professor continued.
"Second: the chronic effects of high-level curses. Some curses lie dormant in the body, gradually altering the host's physiology and magical structure. Documented cases are rare. The Blood Curse is one such example."
Regulus nodded. Nagini.
"Third: certain highly complex rituals. Blood rites, sacrificial rites, ancient covenant ceremonies. The price of such magic sometimes manifests physically. Participants undergo changes in appearance, sometimes temporary, sometimes permanent, depending on the nature of the ritual and how the cost is distributed."
When he reached the next possibility, Slughorn's rhythm slowed.
He lifted his teacup, took a sip, set it down. His gaze settled on Regulus and lingered.
He was looking at this young wizard of extraordinary talent, composed beyond his years, whose understanding of magic ran deeper than it had any right to.
So much like the other one. But only alike on the surface. There was no greed in this boy's eyes, no hunger for power or immortality.
He wasn't fishing for secrets either, wasn't trying to pry anything loose. He was simply asking the kind of question a student would ask.
And besides, Slughorn reflected, the question itself was perfectly sound. Advanced, yes. But it didn't cross any lines.
He pushed the shadow away.
Then he shifted his tone, letting a note of warning creep in. "There's one more possibility. Deeper than any of the others."
He picked up his teaspoon and stirred his cup. "Felix Felicis. You remember?"
"I do, Professor."
"I've only brewed it successfully twice in my life." No boasting in his tone, though a quiet satisfaction was unmistakable.
"During the brewing process, there's a critical threshold. The liquid gold phase. In that instant, the ingredients undergo a complete transformation. Every original property of the raw materials vanishes, replaced by something entirely new."
He set down the teaspoon and met Regulus's eyes.
"Once you pass that point, there's no going back. Nothing you do, no method you employ, can restore the original materials. And if something goes wrong at that threshold..." His finger tapped once against the desk. "It becomes lethal poison. Same cauldron, same ingredients. Past that point, you get either Felix Felicis or death in a bottle. Nothing in between."
Regulus's gaze held steady on the professor's face.
Slughorn went on. "The materials undergo a fundamental phase transition. Afterward, they're no longer what they were. You can't measure them by the old standards, because they possess new magic, new properties, new tendencies, a new structure."
His voice slowed further. "This can happen to a person, too. When a wizard makes a fundamental alteration to himself, those changes manifest outward. Appearance is only the surface. What's truly changed lies much deeper."
He held Regulus's gaze, his words gaining weight. "Some things, once done, can never be undone."
The sentence hung in the air. Then Slughorn lifted his teacup, took a drink, and popped another piece of crystallised pineapple into his mouth, chewing with a contented smile as though none of it had happened.
Regulus sat in his chair, teacup in hand.
An irreversible phase transition. A new analytical framework.
He'd always understood Voldemort's soul-splitting as damage. The logic of damage was simple: something whole, now broken. Broken but still trying to function, requiring compensatory mechanisms to hold together.
Slughorn had offered a different explanation.
What if the splitting of a soul isn't damage at all? Not in the conventional sense. What if it's a phase transition?
He looked up at Slughorn.
The professor was working on his third piece of crystallised pineapple, cheeks puffed out, smiling contentedly. Then he glanced at the clock on the wall.
"Time for dinner, Regulus."
Regulus rose, his tone sincere, and gave a slight bow. "Thank you, Professor. I learned a great deal today. What you said... it helped me more than you know."
Slughorn waved him off with that same easy smile.
Regulus crossed to the door. His hand had just touched the handle when the professor's voice reached him from behind.
"Regulus."
He turned.
Slughorn sat in his chair, the box of crystallised pineapple still open, his office thick with the cloying sweetness of candied fruit and the scent of expensive ingredients.
The smile on the professor's face was the same as always. The same as every day.
But something lived in his eyes. Hope, perhaps. Expectation placed on a young man's shoulders. Or maybe something more.
"Stay away from the things that can't be undone."
Then he waved a hand. "Go on. Go eat."
Regulus gave a small nod and stepped through the door.