Skip to the bottom for personal context on the following ideas: ALSO I RAN OUT OF ROOM ON THIS POST and the formatting is making my numbers and spacing look doofy.
Part A: Transfiguration bridges
Getting started: Partial transfiguration is the most immediately useful thing besides talking, and as I understand it requires wand contact. Harry transfigures a part of his own finger, on a contacting part of the wand blocked from view, and extends the transfiguration slightly to get access to his blood flow. The blood is transfigured into a channel with a covering of false wood on the back of his wand, held on by an exotic fractal surface maximizing van-der-walls forces (gecko tape). The blood faucet at the end of harry's wand is utilized to transfigure an amount of carbon nanotubes with considerable potential energy stored in hairpin folds. The unzippering of the hairpin folds are the (adiabatic) motive force to fire a relatively heavy lance of multiwalled crosslinked carbon nanotubes stippled in nested nanobuds (small appended buckyballs for friction with external objects and linking between layers) at the ground below voldemort's body. This forms a transfiguration beachhead at a desirable location.
The design of the beachhead: The transfiguration beachhead is first used to prepare harry's nuclear option. First, the beachhead would be anchored by growing a fractal network of bundled nanotubes off of a stiff taproot. A multi faceted disk tip of the taproot, with 3 degree facets stacked two high at slightly different angles, disguised with a matte finish the same color and pattern of the surrounding earth, with a bit of dirt on top, like a roof, forms a shape like the word's most covert pop-up sprinkler head. This is the basis of the transfiguration missile battery. Each facet contains a heavy nanotube lance backed by a hairpin CNT held in tension by nanotube zippers. To be clear, the nanotube zippers are fairly extreme nanotubes with nanobuds on them that interlock with nanobuds at regular intervals on the haripin CNTs. This device is unzipped in waves to minimize noise. It should be noted that this device deploying should make a faint, probably undetectable, hissing noise, as the tiny lances break the sound barrier. Voldemort's true form, however, has no external ears, meaning he can only localize sounds by phase and level difference. As Harry's hissing parselmouth and the firing battery are right below and in front of him, both sound sources exist on the same plane as equidistant to V's ears. Thus, the noise is not localized. Harry can fire his transfiguration volleys while vocalizing something with strong sibilance. (wouldn't it be funny if a parselmouth had a lisp?)
Voldy's turret: A bouquet of CNT zippered lances on top of the battery remain in reserve as an offering to Voldy. Part of the bouquet contains three dozen CNTs aimed at an angle that will not hit Voldy if fired directly at him. The angle should be as tight as possible (I'm not sure how high he's floating, but Harry would) for example, if Voldy is 5 meters up and presents a rough cross section no more than one meter in diameter, plus one third for error, gives a minimum angle of about 8 degrees, though 10 degrees would be OK in case voldy drops a few feet or does a broomstick crabwalk in the air or something. These lances are special in that they should be CNT loaded to go about 50 meters up in this case, or otherwise a good bit higher than voldy currently is. mounting this portion on a flex mounted head (like a pyramid with a CNT connection from the tip to the base of the beachhead) aimed by a set of tensioned CNT guy wires is also pretty key, as it allows harry to re-aim if voldy decides to fly around. Harry has demonstrated the ability to transfigure CNTs into shorter CNTs, even under tension, so this provides an aiming method. (The reason I'm not just having harry lengthen a set of fat CNTs instead of using the zipper hairpin springamabobs is I haven't been able to find enough information on how CNTs do under compression, it's likely someone here will know better to make a workable solution.)
Part B: The Nuclear option (No time shenanigans based survival)
The time it might take to get started: Based on the beach head being a bunch of similar parts of low mass arrayed many times, harry may be able to conceptualize and implement the growth in a matter of seconds. This is based on Harry's practice with the feather, growing out simple similar stages all at once. The springs, zippers, lances, harpoon batteries, facets, lattice structure, etc, are all simple components arranged relative to each other in very few ways. I consider it safe to assume that Harry has a level of concentration right now that makes the battles against the omnicidal AI at the end of Summer Wars look like geriatric speed chess. It might take less than ten seconds, depending on how fast the blood flows down harry's wand to start that initial jump to transfiguring dirt.
Some key calculations: Harry's rate of transfiguration based on one hour to transfigure a unicorn: ~ 8 KG per minute, or 133 grams/second, or 133 billionths of a gram per nanosecond.
Carbon nanotube calcs -
carbon double bond distance: 1.3 A single bond: 1.5A Hybrid Bond: 1.39A
The lattice is hexagonal, h = (√3)s gives the height of a single layer from flat-to-flat (the lengthwise, strong axis for a nanotube) on the hexagon, which is about 2.4A. There's an additional carbon bond to connect to the next carbon (I'm referencing cycloparaphenylene) so 3.6A per ring of six carbons is the length. 3.6A is really just a jerk way of saying .36nm, so multiplying things out to a full meter, gives me just under 2.78*10^9 layers of cycloparaphenylene equivalent carbons, which have 6n carbons, where n decides the aspect ratio. Aspect ratio is kind of arbitrary in this case, but I'm going to go with six, since everything else is a hexagon and that makes nesting really interesting.
So, going back to the beachhead, every facet has redundant pair of harpoons, they're spaced out at 3 degrees apart, they have a massive portion at the end so they have a bit of fling and stick capability, and the tethers are a good ten meters longer than they need to be, with intention to go in an arc over the death eaters and just have the strands drape over their shoulders and extended wand hands. 198 degrees of these things (66 facets) should be enough to sextuple-redundant drape most of the death eaters, with some margin for drift in flight. Additionally, 36 single shots for voldemort himself on the tilting turret. 198+36=234 total nanotubes. Interestingly, while I have the numbers up, thats .4ng per meter if they were all bundled together and totally smooth, but with the nanobuds from the hairpin zipper, I calculate it to be between .405 and .44ng, depending on how many hairpin bends are necessary to really whip everything where it needs to go. (harry might need to experiment with the nanoballistics, that's why there are triple volleys)
energy per kg of mass (E=mc^2) = ~90 petajoules or 22.5 megatons of tnt equivalent
If, using the feather principle and starting from the far tips, harry transfigures the carbon into anticarbon at around the rate that he transfigured that unicorn much earlier in the year, and it reacts with an equal amount of matter, harry will release: 1 kilogram / .4 nanograms = 2.5 trillion (2*90 petajoules) / 2.5 trillion = 72K joules
Now, this is spread across all strands. If we include voldy's turret with all strands fired, that's a little over 150 joules per strand per meter vaporized, which means each death eater has some 900 joules creeping up behind them, and voldy has 5400. Now, there's inverse square at work here, so however far the propagation is away reduces the heat significantly. This is particularly interesting because:
Sum of all Masses Per meter: >.4ng Harry's transfiguration speed: ~.133ng/ns light-nanosecond: 0.3 m
Harry's propagation rate is (hopefully) limited by lightspeed. He might be a smidge slower after that Hermione magic saving incident, but I expect all of the energy is going to arrive at once from the perspective of the death eater's backs, and it's going to involve gamma radiation, delivered by the relativistic antimatter det-cord they're wrapped in six different ways. Everyone, especially voldemort, is going to be riding the skin rocket to the center of the meat Z-pinch, except Harry, who will be very red and flying very backwards. I also expect this would wake Hermione, but I suspect she's just been playing possum this whole time.
__CONTEXT__
Okay, so first time commenter here on LW, and I have a LOT of content that I'd like to append to this top comment, figures I'd like to work out for refinement but I'd rather have more eyes on some of it than really polish all of it and put it out for scrutiny like an hour before the deadline.
I came in a bit late and spent most of yesterday evening piecing together a plan, or more specifically, a net steps in a plan on a whiteboard that resembled something like a CYOA map combined with a very deep and messy solomonoff induction network. It was incomprehensible. Trying to put it down in a comment form for EY to read left me feeling dread, horrible dread, which I ignored as best I could. Also, I wasn't exactly sure if my mess of solution modules was an acceptable way to compensate for imperfect information, or just a rule-breaking. I'm also having trouble re-reading to check for consistency, and I didn't really read the official canon past maybe the third book, and I was in elementary school then.. so know that I'm still making a best effort to do this on my own as per EY's suggestion, I just want to do so transparently so more people better equipped to solve the problem can cannibalize parts of my final solution into some dread machine of winning.
Part C: The Best option
Harry needs to convince Voldy that killing him is a bad move, and will lead to a universe that quirrell will not want to live in medium-term, and no quirrell in the long term. (note: I'm trying not to, but I've caught little bits of people on this page that do amazing snake writing. I'm not going to do that because simple english wikipedia can only take me so far.)
The prophecy has been given as little bits involving stars going out, which I think is a separate prophecy than the one with 'the power that the dark lord knows not'. Taken literally, short term, harry can come up with a powerful sacrificial ritual to harness energy that has left a star, medium term, can pull off various feats of stellar engineering like dyson spheres (this should be the name of a vacuum cleaner model), and long term, can just shut down stars and use the matter for more useful purposes, since energy is completely free for wizards anyways. In particular, I was thinking it'd be funny if Harry made his purple light potion, only instead of leaves, he reworks it to use a collapsed star. Additionally, there's a lot of possibilities for techno-horcruxes to inhabit the galaxy. I doubt Voldy has tried horcruxing a sufficiently advanced robot, and IIRC things are set in the late 80s, so there probably won't by any sufficiently advanced computers or robots unless teams of people like Harry are working at it for a while, partially transfiguring novel computronium, and trying to figure out what kind of substrate interactions can be exploited to backup horcruxes to data storage devices, provide sufficient sensory data, and control hardware. If it works, It might be possible to kill one person, make a robo-horcrux of yourself, have the next person kill you, make their robocrux, all the way on to the nth person of the evening, who kills someone, makes a robo-crux, and then time-turns back to be the first to die. Uhh, it's sort of a philosophical issue, but I'm sure Voldy would approve since he's already somewhat exempt from the mass produced techno-immortality circle jerk.
A seperate bit that'll probably interest him is the true patronus. Indifference to killing, indifference to dying, pretty much anybody present except hermione should be incapable of learning
I fell asleep, interfered with my laptop cord, and lost some (a lot) writing, plus polishing. I feel like Ellen Fies in that old mac switch ad. I just learned about 'decision fatigue' from a separate rational fic, but for some reason I just thought I'd be mentally sluggish, not drooling in my chair. I like to think I didn't just bore myself to sleep with my own writing.
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Part D: Talking point, protecting the eternal fun machine I'd like it if Voldy could be seduced into stellar engineering and fast-tracking man's expansion across space. I can think of a lot of arguments that depend on Voldy's utility function, a major (kinda evil slanted) one being that Voldy can't continue making horcruxes without living people around, Voldy can't continue possessing magic without living wizards around, There are nearer-term unpredictable eschaton events than a solar supernova, and also that he values fun, and there are many really good fun theory arguments against floating helpless but mostly immortal in a featureless void for an incomprehensible amount of time. Additionally, Voldy can't reach enough distant objects, even after establishing a portkey system, to keep maintaining even one galaxy indefinitely. Should he need to steer away from a big crunch scenario, or a galaxy swallowing black hole, or an oncoming antimatter galaxy, or some other big end-of-universe spooky, he's doomed unless he maintains enough competent agents spread out everywhere to keep the stuff that entertains him around.
---- I'm going to post now and just keep editing ----
What happens after the atomic win:
Once harry picks himself up and gets to his bag of tricks, the stone, and the time turner, he goes back six hours with hermione, checks on the moon relative to how it was when he left the quidditch game, and sends the perfect patronus to inform dumbledore of the state of things so he can set up an illusion in the mirror room and quit worrying about giving voldy the stone. The cavalry isn't coming, it's already here, and harry has delegated it to trustworthy people.
Harry informs hermione of the true nature of the stone, her existence as a magic creature horcrux experiment, the sacrifice of his own magic he used to reboot her, and how both voldemort and he are tom riddle. She reminds him that he is the heir of slytherin, because they're both up on hogwarts a history.
If time permits (harry can be intercepted en route to the chamber) the perfect patronus goes to get McGonagall, gets her alone, informs her of the anime reference sacrifice hostage situation, and requests her immediate presence in a room off the hallway to intercept past harry. Meanwhile, harry goes off to command slytherin's basilisk, put cute sunglasses on her, a blindfold over that, and transport her to meet with McGonagall in the described place. Once the snake is made safe, hermione is given a rundown of what has gone on and is encouraged to be autonomous or check for understanding until the intercepting phase, when she needs to stop past harry in the hallway, request his glasses and time turner, transfigure the glasses into empty frames, insert the Basilisk lens that McGonagall is maintaining, Insert the [McGonagall wrapped in a future harry's invisibility cloak] lens that McG's maintaining herself as, glue blue tinted flash paper that McG has transfigured into something transparent and thin to the frames behind the lenses. (McG should be offered the chance to just ride in the library room in Harry's bag if she can maintain the transfigurations and perceive well enough to hear a verbal cue [the last thing said by voldemort or any death eater would work, harry can demonstrate the parselmouth if applicable by talking to a serpent] Having her in the bag means she'd be around, and would just so happen to be in his bag right now if he agrees, but only after going in the other room and casting a finite incantatem on his glasses to see if the basilisk is present and find out whether she remembers being triggered to petrify everyone) The prepared glasses are then placed on harry, harry is told to magically affix the frames to the bridge of his nose, hermione requests permission to confund him, instructs past harry (likely as a proxy of future harry) to go back one hour and gather people (such as mad-eye moody and dumbledore) to deal with the hostage situation in the stadium, possibly by carpet bombing the crowd with portkeys immediately after past Harry is out of earshot. Past harry is about to learn about the anime reference blood
There are some likelihoods, like crispy harry has scorched eyeballs and skin and needs to break out the wizard drugs, plus needs to be led around by hermione or draco for a bit. I'm sure hermione, draco, or McGonagall will take care of it before too long, and scorched eyeballs would actually be pretty cool when going to meet your pet basilisk for the first time.
I also think it'd be amusing if Hermione brushed his hair a bit, explanted harry's testicles for safekeeping, dropped in some decoys, and spruced up a bit down there before confunding him. A lot of important evil people are about to see him and grooming is important to people's perception and this is a terrible line of thought and I should feel bad but I don't.