The moment Slughorn heard the question that Snape threw at him, the easy, cheerful look on his face froze solid.
He looked at Snape in shock, his pudgy fingers unconsciously tightening around the stem of his goblet.
Slowly, he licked his lips and asked hoarsely, "What did you just say?"
"I asked whether you know about Horcruxes, sir," Snape said with perfect calm, as if discussing the weather. He flicked his wand toward the door and cast a Muffliato charm before slipping the wand back into his robes.
"What did you do with your wand?" Slughorn's tone had completely changed, gone was the friendliness, replaced by guarded suspicion and defensiveness.
"The Muffliato charm, sir," Snape replied evenly, without haste or anxiety. "To prevent anyone from eavesdropping. A distinguished Auror once taught me: 'Constant vigilance.'"
"All right, all right," Slughorn said impatiently, waving his hand, "but that's not something you ought to concern yourself with, my boy."
He paused, his voice turning stern. "I know nothing about Horcruxes, and even if I did, I wouldn't tell you! Out! And don't let me hear you mention that again!"
"You know nothing about Horcruxes, sir?" Snape said, as though he hadn't heard the dismissal, instead taking a deliberate step closer. "I rather thought that if there were anyone at Hogwarts who could offer informed insight on the subject, it would surely be you. So I decided to ask."
"What?" Slughorn said with a disgusted frown, his tone full of loathing. "Did you? Then you were wrong, weren't you? Dead wrong!"
He barked out the final words and reached out as if to push Snape toward the door.
"Was I wrong, sir?" Snape still refused to move. "That's a pity, because I was going to ask your advice about what to do with a Horcrux I discovered in the castle."
The repulsion on Slughorn's round face collapsed instantly. His features seemed to sink inward; his complexion went chalky and drawn.
"What did you say?" That was the third time he'd repeated those words today, his voice trembling with disbelief.
"I found a Horcrux in the castle," Snape said deliberately, emphasizing every word. "It matched precisely the description I found in a book from the Restricted Section of the library. I'm quite certain it was a Horcrux."
"What book?" Slughorn fumbled at his breast pocket and pulled out a handkerchief to wipe the sweat gathering on his forehead.
"Revealing the Secrets of Advanced Dark Magic, by Olli Blok." Snape answered simply.
"Oh, Dumbledore!" Slughorn burst out, unusually calling the headmaster by name. "That book shouldn't be there!"
"But it is there, sir," Snape said, unmoved. "If I can find it, others can as well, can't they?"
"Perhaps," Slughorn muttered, still dabbing his pale, damp face with the handkerchief. "Though I know nothing about Horcruxes, as your Head of House I could... take a look at the thing you believe to be one. Where is it?"
"If you know nothing about Horcruxes, nothing at all," Snape said, locking eyes with him and stressing the words, "then there's no reason to be so nervous, is there, sir?
"In that case, I won't trouble you. I'll go straight to Headmaster Dumbledore instead."
"I'm not nervous!" Slughorn's voice was unnaturally loud and stammering. "Yes, yes, Albus, you could go to him, but I could help too!"
"I would be very glad of your help, sir," said Snape. "That's precisely why I stayed behind tonight. But we'll need to be honest with each other, won't we?"
"You don't need to be so sharp and curious, Severus," Slughorn grumbled, shoving the handkerchief back into his pocket. "That curiosity of yours will get you into trouble.
"That sort of magic, it's abominable. Truly evil... What exactly do you want to know?"
What do you mean, I shouldn't be so sharp? Snape thought irritably. If you old relics were half as competent, would I have to do all this myself? Watching Pandora run experiments, or having a butterbeer at the Three Broomsticks, either would be far more pleasant than staring at your wrinkled old face.
The entire British wizarding world, waiting idly for a baby to be born, counting on him and his parents' deaths before they could even allow themselves a flicker of relief, and still most of them didn't dare speak Voldemort's name aloud.
Snape felt a flare of bitter frustration. How pathetic. At this rate, if I don't step up, who will?
How can the magical world hope to improve when it's full of flobberworms like these?
This is about survival, for all of us!
Snape refocused his gaze on Slughorn and said quietly, "Out of pure curiosity, sir. I merely wish to know, can a soul be split only once? Or would dividing it into more pieces be... better?"
Slughorn's hand jerked violently. His goblet slipped from his grasp, shattering on the floor with a sharp crack.
"What did you say?!" Sweat was now pouring down Slughorn's face.
His fat, trembling hand fumbled for the handkerchief again, it took him far longer this time to pull it out, and he shakily mopped at the sweat streaming down his forehead.
"You-" Slughorn looked utterly unnerved now. His eyes fixed on Snape, filled with fear, anger, and something like guilt. "You, why are you asking me that-"
", too?" Snape caught the slip instantly. "Someone else asked you the same question before, didn't they, sir?"
"No!" Slughorn blurted, clearly regretting having joined this conversation at all. His voice was cracking. "Don't ask me, I don't know!"
"You haven't answered my question yet, sir," Snape pressed relentlessly. "I was only wondering why someone would leave a Horcrux lying about in a place so easily accessible to others.
"A Horcrux is such a precious magical object, it makes one suspect the creator might have made more than one, doesn't it, sir? What do you think?"
Slughorn stumbled backward a few steps and collided with a chest of drawers. Several bottles on top wobbled precariously and clattered noisily, nearly falling.
"How many times can the soul be divided, sir?" Snape continued. "And the person who asked you that same question before, what did you tell him then?
"According to the great Arithmancer Bridget Wenlock's magical theory, isn't seven the most powerfully magical number? For instance, seven fragments?"