A/N: Finally figured what should happen next, so I've come back to this story. CW: character death.

Chapter Thirteen: Also Allowed to be Smart

Safely back in their secret hideaway, Harry and Hermione stood in silence for a time.

Finally Hermione drew in a deep breath and then released it slowly, tremblingly. Her lips twisted into an unaccustomed bitter half-grimace, half-smile.
"Well, you were definitely right about killing Professor Quirrell. Pity we can't actually touch him, though."

Harry nodded slowly, looking more tired than anything else.
"Yeah. Pity, that. Honestly, I'm just glad we're still alive."

Hermione tilted her head to the side slightly, opened her mouth to answer him. Suddenly, abruptly, her eyes widened. Her face twisted as she gave a gasping sort of half-cry. Her whole body shuddered violently, just once, before dropping bonelessly to the floor.

Harry stared in uncomprehending horror for a split-second, but a moment later he was running forward with his magical first-aid kit already halfway out of his pouch. He pressed two fingers against her throat, desperately hoping for a pulse.

Nothing.

Her skin felt oddly swollen, and slick with sweat that he was sure hadn't been there a moment before. As he drew his fingers away, he noticed an uncomfortable tingling sensation in his fingertips. He held a fingertip to his nose, and sniffed cautiously.

Faint but clear, a sharp smell like dilute vinegar.

The answer seemed to drop into Harry's mind fully-formed, terrible and unarguable. Some quantity of a strong acid, Transfigured into a tiny ice chip or something and then just flicked into Hermione's mouth when her attention was elsewhere. Several hours for that Transfigured substance to circulate and diffuse through her body, until finally the Transfiguration wore off. Whatever quantity of acid, suddenly appearing distributed throughout her body - hence the swelling, the sharp sweat, and of course the near-instant shutdown of her body.

Idiot. Complete, absolute, idiot. Harry had thought of this exact attack, had considered using it himself. He had even spent time trying to think of antidotes, and failed. And yet he hadn't thought to defend against what was in retrospect a fairly obvious threat. And now Hermione was - his brain refused to think it. It fought against years of conditioning, and flinched away from the painful thought. Too painful. No.

He felt his dark side rising, but it too was utterly inadequate to a grief like this. Instead he became simply Harry James Potter-Evans-Verres, but a Harry James Potter-Evans-Verres with nothing of softness or compromise in him. This was his intent-to-kill, his full problem-solving abilities unleashed with no censors whatsoever.

Right. Someone had died. Actually died. No more Batman code. Time to just end this, as quickly as possible. No filters, no censors, just get the job done.

But then, this was Professor Quirrell. How could he possibly take down Professor Quirrell? For that matter, how could he even find him?

Oh, right. Bentham. First problem solved.

How to actually get a shot at Quirrell? Good question. No spell, because Quirrell was just too fast. Actually, he'd need to be completely undetectable in order to have any chance. Right, the Cloak.

What attack? No offensive spell, of course. Transfiguration? High explosives? Muggle explosives didn't really seem up to the task, somehow. It'd have to be something that Professor Quirrell didn't know about.

Antimatter? Harry had thought about this a few times in the past, but had realised quite rapidly that it was a lot less simple than people thought. Antimatter wasn't just this one kind of stuff with the property goes-boom. It was a reflection, an inversion, of normal matter, with all the complexity of that. It would only annihilate neatly if it encountered the exact right kind of normal matter.

Up quarks? Harry wasn't at all sure that that would work, and he also wasn't sure what would happen if it did work. So, no.

Electrons? Low mass, but so tiny that they'd be seriously dense if you packed them tightly. Physics couldn't do that, but Transfiguration could. A cubic centimetre of electrons, densely packed, should have plenty of energy to destroy Professor Quirrell. And Harry himself, of course, but at this point that was simply a necessary sacrifice. Professor Quirrell was now an obvious existential threat to humanity, and Harry was humanity's only hope. Harry was vaguely aware that this explosion would also kill some number of innocent bystanders, but that concern belonged in the world of comic-book heroes. In context of an existential threat to the entire human race, it was regrettable but still ultimately inconsequential that some number of innocent people must die.

Now, the repulsive force between electrons grows more than linearly with respect to both the number of electrons and their closeness to one another. This was a fact known to Harry James Potter-Evans-Verres at this time, at least in theory. It did not occur to him. Instead, just this once, he did the normal human thing; he simply assumed that a cubic centimetre of something non-exotic, no matter what that substance might be, could not actually pose a threat to humanity simply by existing. And electrons were hardly exotic - after all, humans had many whole industries based around manipulating electrons very cleverly.

There was no reason to suppose that Professor Quirrell would destroy humanity immediately, nor even that he would immediately take effective steps to defend himself against discovery by phoenix (if indeed such a defence were even possible). This, too, Harry completely failed to think about.

There also existed many other compelling arguments against taking immediate drastic action. Precisely none of those arguments occurred to Harry; precisely none was allowed to occur to him. Reason was indeed the slave of the passions - Hume had been right about that - and Harry's emotional reality was at this moment still consumed by unthinkable loss.

Harry turned to Bentham, held out a hand. The bird flicked onto the boy's wrist, silent. Harry swirled the Cloak around them both and then thought of Professor Quirrell; a moment later the hideout stood empty.

The first thing Harry noticed was a sense of doom. Extremely faint, but it was there. Idiot! He really, really should have thought of that.

Quirrell was both inactive and asleep - of course, that performance in the Great Hall would have tired him significantly. He was sprawled haphazardly on a bed, diagonally opposite Harry in a fairly large room. There was very little light, and no sign of other people; that was all Harry bothered to notice before focusing on the problem at hand. He might not have much time, before Professor Quirrell might feel the sense of doom and wake up. That could not be allowed to happen.

Harry stared at his ice cube and concentrated on his Transfiguration, articulating inside his head:
"One cubic centimetre of electrons, packed as tightly as possible."

An unfamiliar Transfiguration, but not intrinsically a very difficult one. Not for Harry James Potter-Evans-Verres. It was done in a matter of seconds.

A sense of satisfaction approached Harry's mind as he felt the Transfiguration beginning to complete, and that was the last he knew.

At the tip of Harry's wand, a cubic centimetre of ice became (very briefly) a cubic centimetre of densely-packed electrons. Magic didn't care about conservation of mass-energy, so Harry had not in fact encountered any difficulty Transfiguring a gram of ice into approximately one seventieth the mass of the Moon in electrons. He had merely Transfigured a cubic centimetre of one substance into a cubic centimetre of another substance, and Magic didn't care if the mass had increased by twenty orders of magnitude.

That mass would have severely inconvenienced a great many people, except that it was completely irrelevant beside the total electrical charge on those electrons. Electrons are negatively charged, and therefore they repel each other. Harry had inadvertently concentrated a ridiculous number of electrons in rather a small space, and those electrons therefore proceeded to repel each other with correspondingly ridiculous vigour.

Harry himself simply ceased to exist in any meaningful physical sense, as did his Cloak and everything else that he was carrying. The grief-stricken Harry-in-the-moment had achieved his desired self-destruction, far more effectively than he had ever imagined.

Professor Quirrell's body was likewise utterly unmade within the briefest of instants, as was the room itself.

Within one tenth of a millisecond, the entire city had essentially dissolved.

Another tenth of a millisecond, and the blast radius had reached through the Earth's crust and touched the mantle.

Several hundred kilometres overhead, the Russian space station Mir received approximately two milliseconds of light from the ongoing destruction, before it too was destroyed by the expanding shell of extremely energetic electrons. The two cosmonauts on board knew nothing of this in any case; it was night time in Moscow, and Mir's windows were therefore covered. Four milliseconds had now elapsed since Harry completed his Transfiguration.

Even with a destructive wavefront propagating outwards at half the speed of light, things still take time. The Earth's destruction took eighty-five milliseconds.

The Moon outlived Harry and Professor Quirrell by only a few seconds.

As it happened, Mercury was the nearest extraterrestrial planet; it disappeared a little over ten minutes later.

Another six minutes later, the late Harry James Potter-Evans-Verres did indeed begin to "tear apart the very stars in heaven". The expanding shell of electrons swept the Sun away almost contemptuously, or would have done if emotions applied to simple physics. Even at a radius of a hundred and fifty million kilometres, the electrons still carried more than enough energy density to overpower the star's binding energy. By quite some orders of magnitude still, if truth be told.

In the next twenty minutes, Venus and then Mars were unmade. Each died in darkness, the sun's last rays having outpaced the avenging electrons.

In the end, the local stellar neighborhood at least was saved by a fact that the late Minerva McGonagall had impressed upon generations of students: Transfiguration is not permanent. Somewhere around half a billion kilometres from Earth, that dreadful shell of electrons suddenly reverted to ice.

That initial gram of ice was now spread out around the inner solar system, so for all practical purposes it simply disappeared.

Jupiter, the star that never was, escaped destruction by a quarter-hour.

Three billion kilometres past Jupiter's orbit, a golden plaque drifted peacefully among the stars. Within it lay the last remnant of the man once called Quirrell. Far behind it lay the wreckage of the inner solar system, the last remnant of Harry James Potter-Evans-Verres and his uniquely destructive genius.

Fin.

A/N: This outcome is basically why Quirrell suddenly regards Harry as an existential threat after Hermione's death in HPMOR, and yeah EY was aware of the Transfiguring-electrons idea when he wrote that. I took some ideas and formulas from discussions in several different subreddits, but in the end I did do all the math myself. A cc of densely-packed electrons would contain roughly 1e70 joules of Coulomb energy, which is completely ridiculous. I did calculate how long the electrons would take to reach things (assuming 0.5c speed), including looking up all the actual distances as at April 6th 1992 (except for the plaque which I had to calculate for myself). And yeah that shell of electrons absolutely would tear other stars apart if the Transfiguration lasted long enough. Oh, and I'm assuming that the packing isn't quite enough to bring the strong nuclear force into play. I am not remotely qualified to talk about what would happen in that case.

I know that this ending is weird and kinda unsatisfying, but I think it makes sense for a meta-fanfic like this one. We already have HPMOR itself as a story that follows narrative conventions, which leaves this story free to explore the entirely realistic prospect of sudden catastrophic failure. I didn't make anyone act uncharacteristically in order to bring this about, and I didn't use anything that wasn't already available in the story (except for Transfiguring electrons instead of up quarks like Harry originally wondered about).

Sincerest thanks to my beta reader AnimeKitty47, and to everyone else who reviewed this story. This was my first attempt at writing fanfiction, for that matter it was my first substantial attempt at writing fiction of any kind, and honestly I'm just so glad I finished it in the end.

-Pat