Author's note: This story was originally written in response to a request by kk bk, and posted on my main, "Qoheleth" account. After working at it for some time, though, I decided that kk bk's original vision and my writing style just didn't match very well, so I removed it from that profile. I still rather liked what I had, though, so I decided to post it here – which means that it, like most of my unfinished stories on this account, is up for adoption. Make me an offer.

Disclaimer: Recipe for an AU: Take a universe by J. K. Rowling, an idea by kk bk, a title from the Book of Exodus, and a picture from NASA. Mix together and let simmer for 2 years, or until a philosophical analysis inserted in the center comes out clean. Serve when cool.

"I'm cold," Maia Cranston complained as she and her fellow third years trooped up the steps of the Astronomy Tower. "Why can't the greatest wizarding school in Britain be somewhere warm? Like Gibraltar. What's wrong with having it in Gibraltar?

"I don't think Gibraltar was part of Britain when Hogwarts was founded, Maia," said Felicia Park-Place with a small smile. "Remember, it was in the tenth century; the Normans hadn't even invaded England yet."

"Well, still, they didn't have to put it this far north," said Maia. "You've seen how high Polaris is every night; we must be at least north of the Pennines…"

"Very good, Miss Cranston," said Professor Sinistra, pausing and turning to smile at her from the head of the queue. "Five points to Ravenclaw for applied knowledge."

Maia blushed at being overheard, but there was a pleased smile on her face; though quite intelligent, she was not one of the more conspicuous students, and earning points was something of a novelty for her.

"Knowledge, moreover, that will come in useful tonight," Professor Sinistra added. "The cosmic event that you are all about to witness is occurring right near Polaris."

"Come on, Professor, kill the suspense," said Asclepius Wilson. "Why can't you just tell us what we're going to see?"

"I'll bet it's a nova," said Arachne Dentor. "That's the ultimate cosmic event, isn't it?"

"Or a comet," said Anthony Zeno. "Isn't there a comet that a couple of Americans discovered last year, and it's going to show up in the sky soon?"

"No, that won't get here till next year," said James-Helvidius Greggson. "And it wouldn't be a conjunction of planets, either, if it's in Ursa Minor. I'll bet Arachne's right about the nova."

"Course he would," Sabrina Barker whispered to Felicia. "He backs up everything Dentor says, the turncoat."

"They're friends," Felicia whispered back. "They've been friends since they were kids. What do you expect?"

"I expect a Ravenclaw to have proper house pride," said Sabrina loftily. "We're never going to win the Cup if our best Housemate spends all his time doting on some hemophilic Slytherin princess."

Felicia didn't bother to respond. She knew that Sabrina had been sweet on James-Helvidius ever since their first class together, and she knew the sound of jealousy when she heard it. Besides, they were getting close to the top of the Tower; any second now, Professor Sinistra would say…

"All right, ladies and gentlemen, time to switch off your chatterboxes," said Professor Sinistra. "Miss Lovegood, will you get the door, please?"

Luna Lovegood obligingly tugged on the rope above her head; the stairs tumbled down into place, and in a matter of seconds the assembled Ravenclaw and Slytherin fourth years stood atop the Astronomy Tower.

Several students shivered beneath their cloaks. It was a few days since the vernal equinox, but, at Hogwarts, there was still a distinctly wintry nip in the air, largely because the weather had been so damp lately. Indeed, that evening was the first time in nearly a fortnight that the sky had been free of clouds. Several students remarked on this.

"Rather convenient, isn't it, Professor?" said Asclepius. "The night of your big event, and all of a sudden everything clears up."

"Very convenient," said Professor Sinistra with a smile. "That was why I petitioned the Headmaster to do it."

Asclepius blinked. "You mean that you had Professor Dumbledore move the clouds aside?"

"Just for tonight," said Professor Sinistra. "One doesn't want to cause any serious disruption of the climate; the Ministry, you know, very much frowns on that sort of thing. But I don't think it will do the North Sea any harm to experience a few hours of unusually heavy rainfall."

Libby Sylvain whistled. "You really went all out, didn't you, Prof?" she said. "This must really be some cosmic event you've cooked up."

Professor Sinistra, who disapproved of slang, gave her a tight-lipped smile. "Well, of course it was Nature, not I, that – ah – 'cooked it up'," she said. "But see for yourself. Miss Cranston, if you would be good enough to point out Ursa Minor for us?"

Maia turned obediently to the north, and raised her head to the region of sky where Polaris invariably twinkled over the ancient castle. Then her eyes widened, and it was some seconds before she could whisper, in a tone of wonder, "Professor, what is that?"

Her fellow students followed her gaze, and were likewise startled at what they saw. Lying between the Dippers, where nothing, assuredly, had been before the clouds had set in, was a brilliant green light, brighter than anything else in the sky except the moon; it was surrounded by a kind of veil that seemed to both soften and magnify its radiance, and, behind it, a luminous tail extended nearly a finger's length across the sky.

"Well, what do you know," said Sabrina. "Looks as though you were right after all, Anthony."

Anthony smirked. "Well," he said with a sidewise glance at James-Helvidius, "we're all right sometimes – and we're all wrong sometimes, loath though some of us may be to admit it."

James-Helvidius, who was frowning up at the comet with a look of intense puzzlement, gave no sign of having heard this barb. Arachne, however, had, and was goaded into attack. "I never mind admitting that you're wrong, Zeno," she said, in the tone of arsenic sucré that was her particular specialty.

Anthony reddened. "Listen, Princess Romanov," he said, "we don't need to hear from…"

"Right there, for instance," said Arachne, triumph gleaming in her eye. "I don't have the least compunction about pointing out that the feminine form is Romanova."

A murderous look passed over Anthony's face. "You know, you're pretty brave for someone who goes to the hospital wing every time she stubs her toe," he spat. "Why don't the two of us step outside after class, and I can introduce you to a few tricks my uncle taught me to keep little inbred pure-blood tight-arses in…"

"Eh-hem!"

Anthony blinked, and seemed to suddenly remember where he was. He glanced up sheepishly at Professor Sinistra's icy glare, and murmured a vague apology.

The Astronomy mistress was far from appeased. "Remember this moment the next time someone implies that our House has a monopoly on boorishness, ladies and gentlemen," she said, glancing toward the Slytherin side of the class. "Now, then, let's come to practical measures. Twenty-five points from Ravenclaw for threatening a fellow student's well-being; fifteen more for taking verbal advantage of another's misfortunes; ten for the use of language unbecoming a Hogwarts student; ten from Slytherin for provoking him, Miss Dentor; and," she added, turning to James-Helvidius, "five to Ravenclaw for demonstrating how a civilised human being responds to detraction, Mr Greggson."

James-Helvidius blinked, and seemed to come out of a trance. "Oh, sorry, Professor," he said. "Did you ask a question?"

Professor Sinistra smiled ever so faintly. "No, Mr Greggson," she said. "I did not."

"Oh," said James-Helvidius. "Well, sorry, anyway. I guess I just got a bit lost in my thoughts. I could have sworn that Hale-Bopp wasn't due until next year."

"It isn't."

James-Helvidius blinked. "Then what's that?" he said, with a nod toward Ursa Minor.

"That," said Professor Sinistra, "is C/1996 B2, otherwise known as Comet Hyakutake. It was discovered not quite two months ago by a former photoengraver named Hyakutake Yuji. I'm a bit surprised that none of you knew of it, actually; there was a small but quite noticeable piece about it in the Daily Prophet last month."

"I don't take in the Prophet," said James-Helvidius. His face was glowing with excitement (in contrast to Anthony's, which had gone even redder than before at this unexpected vindication of James-Helvidius's expertise). "So there will be two comets passing through the inner Solar System between now and next spring?"

"So it seems," said Professor Sinistra. "Hopefully, the sky will be a bit clearer for C/1995 O1's approach; I doubt that Professor Dumbledore will be inclined to permit a second weather-witching for this purpose. But let's not worry about that now. Mr Wilson, Miss Barker, could you help me set up the telescopes on this side, please?"

In a few minutes, everything was set up, and the class got down to the serious business of observing and analysing Hyakutake's discovery. For the remainder of the class time, no words were spoken aloud except by Emily Kingdomtide (who had a habit of murmuring her notes to herself as she took them).

But, though nothing was said, a great deal was thought. Some of it concerned the comet; some of it concerned the recent altercation; some of it concerned other matters entirely.

"Heaven bless you, Albus Dumbledore. You hire centaurs, but you don't let their inexplicable aversions to sky-clearings keep you from doing an old colleague a favour. That's what I call the rational, well-balanced mind."

"'Princess Romanov', honestly. You'd think even a Ravenclaw could come up with something better than that."

"I can't believe that Mancunian guttersnipe just lost us forty-five points. Someone needs to do something about him."

"So this is what the Destiny Star looks like. Dad will be so pleased to know that I got to see it… but I mustn't ask it for anything. We promised Metron we wouldn't."

"Proud celestial prodigy, sailing in a starry sea…"

"Oh, drat. That's the third time I've dropped my quill over the wall this term. Good thing I brought a spare this time."

"Just try it again, Dentor. Sometime when Sinistra's not around. Just try."

But none of the thoughts that passed through their minds – or, indeed, the minds of anyone at Hogwarts on the evening of 25 March 1996 – concerned the radical alteration that was about to be made to the very fabric of history, both past and future. Of all the souls at Hogwarts, only one so much as imagined that such a thing was possible, and no-one – not even the two fifth years who would set it in motion, and who would be more transformed by it than anyone else – had any notion of the form it would take.

This is the story of that alteration. This is the story of Hydra Malfoy.