Harry Potter and the Methods of Rationality

Harry promptly reached into his robes and drew out a parchment, unfolding it and spreading it across the golden table. "I've taken the liberty myself, actually," Harry said. He'd spent some careful hours in the Hogwarts library with the law books available. Thankfully, so far as Harry could tell, the laws of magical Britain were charmingly simple by Muggle standards. Writing that the original blood debt and payment was cancelled, the Potters' wealth and all other vault items would be returned, and the remaining debt annulled, all with no fault to the Malfoys, was only a few more lines than it took to say out loud. "I had to promise my keepers not to sign anything you gave me. So I made sure to compose this myself, and sign it before I left."

Draco emitted a choked laugh.

Lucius read through the contract, smiling humorlessly. "How charmingly straightforward."

"I also promised not to touch a quill while I was in Gringotts," Harry said. He reached into his robes again and drew out a Muggle pen, along with a sheet of normal paper. "Will this wording be all right?" Harry rapidly scribbled down a legal-sounding statement to the effect that House Potter didn't hold House Malfoy responsible in any way for Hermione Granger's murder and didn't believe they had anything to do with it, then held up the paper in the air for Lord Malfoy's inspection.

Lord Malfoy looked at the paper, rolled his eyes slightly, and said, "Good enough, I suppose. Though to have the proper meaning, you should use the legal term indemnify rather than exonerate -"

"Nice try, but no. I know exactly what that word means, Lord Malfoy." Harry took his parchment and began copying down his original wording more carefully.

When Harry was done, Lord Malfoy reached across the golden table and took the pen, looking at it thoughtfully. "One of your Muggle artefacts, I suppose? What does this do, son?"

"It writes without needing an inkwell," Draco answered.

"I can see that. I suppose some might find it an amusing trinket." Lucius smoothed the parchment contract over the table, then set his hand by the line for signatures, tapping the pen thoughtfully on the starting spot.

Harry wrenched his eyes away, up to Lucius Malfoy's face, forcing himself to breathe regularly, not quite able to stop his muscles from tensing.

"Our good friend, Severus Snape," said Lucius Malfoy, still tapping the pen on the line awaiting his signature. "The Defense Professor, calling himself Quirrell. Now I ask again, who is your third suspect, Harry Potter?"

"I would strongly advise that you sign first, Lord Malfoy, if you're going to do so anyway. You will benefit from this information more if you do not think I am trying to persuade you of something."

Another humorless smile. "I shall take my chances. Speak, if you wish this to continue."

Harry hesitated, then said evenly, "My third suspect is Albus Dumbledore."

The tapping pen stilled on the parchment. "A strange allegation," Lucius drawled. "Dumbledore lost much face when a Hogwarts student died within his tenure. Do you suppose that I will believe anything of him, only because he is my enemy?"

"He is one suspect among several, Lord Malfoy, and not necessarily the most plausible. But the reason I was able to kill a full-grown mountain troll was that I had a weapon which Dumbledore gave to me, at the start of the school year. It's not strong evidence, but it's suspicious. And if you're thinking that murdering one of his students is not Dumbledore's style, well, the same thought had occurred to me."

"It's not his style?" Draco Malfoy said.

Lucius Malfoy shook his head in a measured, careful movement. "Not quite, my son. Dumbledore is particular in his evils." Lord Malfoy leaned back into his chair, and then sat quite still. "Tell me of this weapon."

"I am not yet certain I should go into details about that in your presence, Lord Malfoy." Harry took a breath. "Let me be clear on this. I am not trying to sell you on the idea that Dumbledore is behind this, just raising the possibility -"

Then Draco Malfoy spoke. "The device Dumbledore gave you - was it something to kill trolls? I mean, just trolls? Can you tell us that?"

Lucius turned his head to look at his son with some surprise.

"No..." Harry said slowly. "It wasn't specifically a sword of anti-troll slaying, or anything like that."

Draco's eyes were intent. "Would the device have worked against an assassin?"

Not if they had shields raised. "No."

"A fight in school?"

An expanding rock in the throat is inherently lethal. "No. I don't think it was meant for use against humans."

Draco nodded. "So just magical creatures. Would it have been a good weapon against an angry Hippogriff, or something like that?"

"Does the Stunning Hex work on Hippogriffs?" Harry said slowly.

"I don't know," said Draco.

"Yes," said Lucius Malfoy.

Compared to trying to target a Wingardium Leviosa and Finite Incantatem - "Then a Stunning Hex would be a better way of dealing with a Hippogriff." Put that way, it did seem increasingly like a Transfigured rock was an optimal weapon only against a flesh-and-blood magical creature with spell-resistant skin. "But... I mean, it might not have been intended as a weapon at all, I used it in a strange way, it could have just been a crazy whim -"

"No," Lucius Malfoy said lowly. "Not a whim. Not coincidence. Not Dumbledore."

"Then it's him," Draco said. Slowly Draco's eyes narrowed, and he gave a vicious nod. "It's been him