“You don’t know the half of it, Rosmerta,” said Fudge gruffly. “The worst he did isn’t widely known.”

“The worst?” said Madam Rosmerta, her voice alive with curiosity. “Worse than murdering all those poor people, you mean?”

“I certainly do,” said Fudge.

“I can’t believe that. What could possibly be worse?”

“You say you remember him at Hogwarts, Rosmerta,” and murmured Professor McGonagall. “Do you remember who his best friend was?”

“Naturally,” said Madam Rosmerta with a small laugh. “Never saw one without the other, did you? The number of times I had them in here - ooh, they used to make me laugh. Quite the double act, Sirius Black and James Potter!”

Harry dropped his tankard with a loud clunk. Ron kicked him.

“Precisely,” said Professor McGonagall. “Black and Potter. Ringleaders of their little gang. Both very bright, of course -- exceptionally bright, in fact -- but I don’t think we’ve ever had such a pair of troublemakers --”

“I dunno,” chuckled Hagrid. “Fred and George Weasley could give ‘em a run fer their money.”

“You’d have thought Black and Potter were brothers!” chimed in Professor Flitwick. “Inseparable!”

“Of course they were," said Fudge, “Potter trusted Black beyond all his other friends. Nothing changed when they left school. Black was best man when James married Lily. Then they named his godfather to Harry. Harry has no idea, of course. You can imagine how the idea would torture him.”

“Because Black turned out to be in league with You-Know-Who?” whispered Madam Rosmerta.

“Worse even than that, m’dear...” Fudge dropped his voice and proceeded in a low rumble. “Not many people are aware that the Potters knew that You-Know-Who was after them. Dumbledore, who of course was working tirelessly against You-Know-Who, had a number of useful spies. One of them tipped him off, and he alerted James and Lily at once. He advised them to go into hiding. Well, of course, You-Know-Who wasn’t easy to hide from. Dumbledore them that their best chance was the Fidelius Charm.”

“How does that work?” said Madam Rosmerta, breathless with interest. Professor Flitwick cleared his throat.

“An immensely complex spell,” he said squeakily, “involving the magical concealment of a secret inside a single living soul. The information is hidden inside the chosen person, or Secret-Keeper, and is therefore impossible to find -- unless of course, the Secret-Keeper chooses to divulge it. As long as the Secret Keeper refused to speak, You-Know-Who could search the village where Lily and James were staying for years and never find them, not even if he had his nose pressed against their sitting room window.”

“So Black was the Potters’ Secret-Keeper?” whispered Madam Rosmerta.

“Naturally,” said Professor McGonagall. “James Potter told Dumbledore that Black would rather die than tell where they were, that Black was planning on going into hiding himself. .. and yet Dumbledore remained worried. I remember him offering to be the Potters’ Secret-Keeper himself.”

“He suspected Black?” gasped Madam Rosmerta.

“He was sure that somebody close to the Potters’ had been keeping You-Know-Who informed of their movements,” said Professor McGonagall darkly. “Indeed, he had suspected that someone on our side had turned traitor was passing a lot of information to You-Know-Who.”

“But James Potter insisted on using Black?”

“He did,” said Fudge heavily. “And then, barely a week after the Fidelius Charm had been performed ---”

“Black betrayed them?” breathed Madam Rosmerta.

“He did indeed. Black was tired of his double agent role, he was ready to declare his support openly for You-Know-Who, and he seems to have planned this for the moment of the Potters’ deaths. But as we all know, You-Know-Who met his downfall in little Harry Potter. Powers gone, horribly weakened, he fled. And this left Black in a very nasty position indeed. His master had fallen at the very moment when he, Black, had shown his colors as a traitor. He had no choice but to run for it --”

“Filthy, stinkin’ turncoat!” Hagrid said, so loudly that half the bar went quiet.

“Shh!” said Professor McGonagall.

“I met him!”

Rest of this passage coming as soon as I have time to type it. :-)