[Author's note: Arthur C. Clarke's third law states said that any sufficiently advanced technology is indistinguishable from magic. I imagine it works the other way, too.]
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This was it.
The battle had gone poorly – most of his Death Eaters were dead, stunned or captured. His giants had fled. The werewolves were systematically being cut down. Even faithful Bellatrix had been killed – destroyed by a blood-traitor housewife, of all people.
But it didn't matter – he was Lord Voldemort! Once he was triumphant, it would be easy to rebuild his forces. And he would triumph, for even on his own, none of these blood-traitors and children could possibly take him on.
Well, there was one possibility.
As the battle raged on around him, Potter finally dared to face him, although he was busy prattling on about Snape, and Dumbledore, and Draco Malfoy. And the Elder Wand. Could Potter be right? The Wand had not been responding to him as legend said it should – spells were working improperly, his Killing Curses were far less lethal than they should have been – and it was getting worse as the battle wore on. Could Potter be right?
No matter – even with a child's training wand he should be able to defeat Potter easily. A malfunctioning wand in his hand was no problem for the greatest wizard of all time!
With spells being cast in all directions, and for so long, he could feel the magic in the air, pulsing all around him, growing stronger and wilder in this, surely the greatest battle since Grindelwald. He raised the Elder Wand at the same time Potter raised Draco Malfoy's wand, and as Potter cast the Disarming Charm – how typical, even now – he cast the Killing Curse at his nemesis, felt the magic begin to flow through him, and then... nothing.
No green light, no Potter dead at his feet. He feared the Deathstick had truly failed him, until he realized he still held it in his hand. Potter's spell had failed as well! He then noticed there were no spells being cast by anyone at all, and he could no longer feel the magic in the air, the magic in his veins.
Silence enveloped the battlefield, which made the triumphant "YES!" from Potter's mudblood henchwoman all the more piercing.
"What have you done?" Lord Voldemort shouted. "Where is the magic?"
"I haven't done anything," Potter said. The boy was more composed than Lord Voldemort himself was, which made him worry all the more.
He felt every one of his seventy-one years. He had never felt so weak, so old, but it was still there. He could still feel vestiges of the magic – little pulses that he was sure no one else would be able to feel, all of them being so inferior to him in matters magical. It was still there, the magic, trying to come back, it was just inaccessible. Unavailable. Still, he was certain it wouldn't be long until he could cast the Killing Curse once more and end the life of the miserable wretch in front of him.
He laughed. "I don't know what your plan was, boy, but it won't work. You're as helpless as I am until the magic comes back, and when it does, you die. Wait, stop," he said fearfully, backing up slowly, oh so slowly. Why wouldn't his body respond? "Put that down! You're a wizard, damn you! Stop!" he shouted, feeling every moment of his age, until he didn't any longer.
"... so then I bashed him in the head with a rock," Harry said. "I think he died with the first hit, but I got in a couple more for good measure. When it came back a few minutes later, he was still dead."
"Amazing!" said Dumbledore's portrait.
"Pathetic," said Snape's. Even mostly destroyed, the castle still possessed enough magic to create a portrait of the deceased Headmaster. "Six years of education here, and your 'triumph' comes from something a Neanderthal could have done. Hitting an old man on the head with a rock."
"Severus, really," Dumbledore's portrait said. "I knew something had happened – it was as if I suddenly did not exist. I never would have guessed, though, that magic could fail utterly, even for only fifteen minutes. Was the phenomenon worldwide?"
Harry shook his head. "Apparently it only happened within a circle of about ninety miles radius from here. Hermione expected it to be localized, but even then it was more widespread than she thought."
"What do you mean, Granger expected it?" Snape's portrait said, disbelieving.
Harry explained. "Well, when we were hunting for Voldemort's horcruxes, we had a lot of time on our own. For a while it was just Hermione and me. We'd tell stories about when we were kids to keep from being too bored. I told her how my cousin Dudley would just rage whenever his Nintendo failed – that's a video game system. Electronic entertainment. She laughed, and said how frustrated her parents would get when the power went out at work, or when they couldn't get onto their bulletin board service on their computer. I asked how that happened, and she said it was when there were too many users trying to access it at once."
Harry could see that the two portraits didn't entirely follow him, but got the gist of what he was saying. "So I asked if that could ever happen with magic. And it's like a light went off in her head. She said you wouldn't think so – there's lots of students using magic in Hogwarts all the time. But it's usually low-grade, not very intense magic. Not battles, or life-and-death kind of stuff. She said that most wizard battles would barely count as skirmishes in Muggle wars, because there's so few wizards, but if you got enough of them in one place, having a battle, it could... well, neither of us knew what you'd call it, but the Muggle equivalent was overloading or crashing the network."
"But such a thing never happened against Grindelwald," Dumbledore's portrait said, "and those were some great battles. Why now?"
"Hermione thinks it's because Hogwarts uses so much magic already that it just completely overloaded the local magic, and 'poof', the system went down. Of course, Luna said that the nargles probably hid all the magic for a laugh," Harry said with a smile.
His eyes twinkling, Dumbledore's portrait said, "We should probably investigate both possibilities, Harry."
"There's one thing I don't understand," Snape's portrait said. "Sybill talked about the 'power he knows not'. I'd hardly believe the power the Dark Lord knew not was a rock!"
Dumbledore's portrait said, "It could be that I did not hear the entire prophecy, because it seems that what happened was the power Tom knew, being magic, not working. It was very hard to hear in the Hog's Head that day. My brother has rather a raucous clientele."
Snape's portrait scoffed.
"I have to go, Headmaster. Headmasters, sorry. The Department of Mysteries is going to want to talk to Hermione and me. Something about a job. I'll stop by and visit when I can," Harry said, waving goodbye.
"Don't trouble yourself," Snape's portrait said with a sneer. "A rock. Really. Why do we bother teaching them at all?"