Prologue
The one with the power to vanquish the Dark Lord approaches ... born to those who have thrice defied him, born as the seventh month dies ... and the Dark Lord will mark him as his equal, but he will have power the Dark Lord knows not ... and either must die at the hand of the other for neither can live while the other survives ... the one with the power to vanquish the Dark Lord will be born as the seventh month dies ...
Magic, as any pre-Merlin witch or wizard would tell you, turned your will and belief into reality. And though rituals existed, and certain things of nature had their properties, and most magical practitioners could feel magic, in the end, it was through language (which, after all, was how all humans, magical or not, expressed themselves) that all magic was based.
An experienced witch or wizard had attuned to the magic that ran through the earth so finely that he or she could speak a few words, using a piece of wood containing a part of a being of magic for a conduct, and, if it was small enough, it would happen. (For larger magics, nature had its magical inertia, and it had to be persuaded.) There was further a subset of witches and wizards with a strange connection to magic, such that, even without the use of a conduct, they could seem to alter the universe to their desires.
Of course, as words were power, all pre-Merlin magical children grew up learning to treat words extremely respectfully. That is, all magical children born to magical parents. This unsurprisingly lead to unfortunate magical accidents from what were then called magbobs (later Muggleborns), but it was really only after Rosaline Addinell, a gifted but unfortunately temperamental witch, said a curse and nearly destroyed Hogwarts that the subject of whether magbobs were qualified to handle magic really came up.
That was until a brilliant if relatively magically untalented wizard called Merlin (he had no other name, even then) with a grand ambition to stop this problem found a solution. He invented a system of specific spells to be taught to all children that could do certain things and would be based in an older language (he chose Latin). This effectively separated words of magic from words of everyday speech, eliminating most of the problems. But even that was not enough for him. He realized that even that would not be enough to safeguard them from the magicals with the strongest connection to magic. After years of exploring magic he realized that these people's words became a conduct for magic, whether magic wanted it or not, and developed a method to prevent them from doing this except at magic's request. Because what these people said became reality, they were eventually dubbed Seers. (It only took a few centuries for people to forget exactly what Merlin had done.)
But enough of history! When a wizard who called himself Lord Voldemort, through his unbalanced explorations into the deepest depths of magic, threw the universe off-kilter, it naturally had to rebalance. It chose a Seer named Sybill Trelawney to right itself.
And so, on October 31st, sometime between 8:00 and 9:00 pm, the Potters found themselves with an unexpected guest.
On that day, it so happened, they were not alone. Though in some worlds they were not, in this particular one Lily, now Potter, and Alice, now Longbottom, had been inseparable throughout Hogwarts (though Alice could not understand what Lily saw in Severus). When the prophecy was revealed, it was only natural that Lily and Alice visited each other frequently.
In the spirit of Halloween, James had charmed Alice to look like Frank, Frank to look like James, James to look like Lily, and Lily to look like Alice. Lily tried to explain that dressing up was not supposed to be like this and Frank protested that he really didn't want to look like James, but in the end they had all laughed and tried to adapt to wearing other bodies.
"The boys are asleep," Lily-looking-like-Alice said.
"Are you sure?" Frank-looking-like-James asked worriedly. "Neville's been known to fake it."
James-looking-like-Lily gave him an incredulous look. "You're kidding. Sweet, innocent little Neville, pretending to be anything?"
"That's what you said about Harry," Lily-looking-like-Alice pointed out.
"He loves plants," Alice-looking-like-Frank explained. "So at night he'll try to crawl out of his bed and out the window to see the plants that only open when the moon shines. He made it all the way into the greenhouse without us knowing a thing once."
Lily-looking-like-Alice was halfway through a laugh when the door was blasted open.
Voldemort.
They seized their wands immediately, and moved as one. Frank-looking-like-James and James-looking-like-Lily ran forwards immediately to combat Voldemort. Even as terrified as Lily-looking-like-Alice and Alice-looking-like-Frank were, they exchanged a look that clearly said, "Men."
The two of them took up stations behind them, knowing that four people dueling one was worse than two in such a small space, grimly ready to be the last line of defense for their children.
It was all they could do to not join in the fight, but their common sense (and Alice's training) held. Frank-looking-like-James and James-looking-like-Lily were excellent duelers in their own right, but neither of them were particularly comfortable in their fake bodies (that even James could not dispel until midnight) and even normally, were no match for Voldemort, who had spent decades delving into such arcane magics that Lily was impressed against her will. Horrified, but taking notes.
There was a loud bang and both Frank-looking-like-James and James-looking-like-Lily were blasted aside. A wave of his wand and Frank-looking-like-James was dead (Lily-looking-like-Alice shot a quick, temporary silencing charm at Alice-looking-like-Frank when she looked like she was going to scream) but they were both shocked to see Voldemort allow James-looking-like-Lily to clamber to his feet, clutching his wand tighter.
"You cannot win," Voldemort said, his silky voice soft. "Step aside, you silly girl. My business is not with you."
The look James-looking-like-Lily wore was not out of place on Lily's visage - the sharp, narrowed eyes, fury in every line. When he spoke, it was low and sibilant. "You will never touch -"
That was all he got before Voldemort flicked his wand carelessly, blasting him out of his way. Lily-looking-like-Alice and Alice-looking-like-Frank barely had time to wonder that Voldemort had killed who he thought was James, a pureblood (even if only barely and not from a "noble" family) but left who he thought was Lily alive before he noticed them. His eyes widened imperceptibly, revealing his shock at their presence.
His lips curled into a smile. "Both boys, here at once? I hardly know which one to kill first."
Faster than the eye could see, Lily-looking-like-Alice and Alice-looking-like-Frank slashed their wands, but the spells dissolved before they could reach him. With a twirl and an incantation, Lily-looking-like-Alice made the dust on the floor fly up to blanket his eyes, nose, and mouth. Voldemort struggled for a moment to duel blindly before he forced the dust off and at them. Lily-looking-like-Alice flicked her wand and it formed a blob that attacked Voldemort while Alice-looking-like-Frank shot rings of fire at him, which he parried and returned in the form of a fire-serpent that ate the blob of dust. The fire-serpent hissed as it hit the shield of water Alice-looking-like-Frank put up and Lily-looking-like-Alice immediately sent a modified bubble-head charm at Voldemort.
Spells flew thickly through the air, and through the corner of her eye Lily-looking-like-Alice noticed James running toward the nursery as silently as possible. She immediately had to duck a killing curse and returned with a switching spell to switch his heart with a knife. Like the others, it was blocked seemingly effortlessly and Voldemort uttered a word they had never heard of, which an instinctive shield blocked. Their wands flashed through spell-patterns and all duelers could feel the magic in the air.
But the real difficulty with fighting Voldemort was not speed or power. It was that, while others fought with spells and transfigurations, Voldemort (like Dumbledore), seemed to be able to use pure magic, the only description that seemed plausible. They felt whatever he had done before it came, but it passed through their shields to pick them up and hang them in the air helplessly.
"I do not," he said finally, "want to spill noble blood more than I have to. Propagate, continue your line, for your children will be of noble stock."
They felt themselves drop, but could do nothing before the world went black.
Voldemort continued on, his way now unimpeded. When he entered the small nursery, however, he found that Lily-who-was-actually-James was already in there.
She was a nuisance, but Severus begging him for her life gave him pause again. He knew, after all, that magical ability was sometimes indiscriminate of blood, and by all accounts, Lily Potter had that. From Severus's accounts, she had had control of her "accidental" magic from a young age, much like himself.
"Move aside," he repeated himself. "My business is not with you."
"Don't touch them!" She sounded desperate now, clutching her wand tighter and throwing herself protectively in front of the two boys in the crib.
"Move aside, you silly girl!"
Impatient when she refused, he killed her, and she crumpled down, lifeless.
Finally, he reached his goal - or, perhaps it was better to say, goals. The two boys, one dark haired, the other lighter, were sitting, foreheads pressed together, looking at something the lighter haired one was holding which was clearly more interesting than whatever game the big people had been playing. Voldemort thinned his lips, annoyed at being ignored, but as he raised his wand to force them to look at him, it occurred to him that they were close enough that he could take them both out with a single curse. A smile curved his lips. How fitting.
Intent on his victims, he did not notice the magic that was building up, so thick that you could cut it with a knife, centered on the oblivious boys.
"Avada Kedavra!"
Green light splayed on their adjoined foreheads, and then pain, more than anything else, as Voldemort was ripped from his body, part of his soul tearing off. He fled, less than ghost but more than dead, as an explosion shook the house.