CURRENTLY UNDERGOING HEAVY REVISION - Most of it is likely to be scrapped and redone from the ground up.

Disclaimer: Joanne Rowling brought Harry Potter into our world. Eliezer Yudkowsky found a nearby timeline (which this story branches off from) within the space of possible books surrounding Harry Potter. Robert A. Heinlein codified the idea of "the World as Myth." The cover image contains foreground elements from an unnamed vanitas painted by Simon Renard de St. André, and the background was created by Google's Deep Dream Generator.

Author's Notes: This is a sequel to Harry Potter and the Methods of Rationality, and it begins in medias res during HPMOR's ending. It isn't entirely necessary to have read HPMOR in order to follow the plot of HPIO. However, HPMOR is pretty great, and you're gonna have the entire ending of it spoiled if you try to read HPIO first, so, y'know, maybe consider reading HPMOR if you haven't already. Also, in the interest of full disclosure I am obligated to inform you of the fact that this is a Mega Crossover, or a Massively Multiplayer Crossover, or a Multicross, or whatever the hell you wanna call it. "A sequel to HPMOR? And on top of that it's a crossover? There's no way in hell this isn't gonna turn out to be a rolling dumpster fire." I know this seems like an absurd premise for a story. Trust me, I know. But this is not going to go the way you think.

Nothing is infinite. Everything has an end. Mors fontis Omnia.

The sound of gunfire rang in her ears as she reached into her pouch and drew forth the wax-sealed paper, on the surface of which was inscribed simply the number 42. She broke the seal and began to read...

HARRY POTTER

AND

THE IRRATIONAL ODYSSEY

Book One: Everything Not Forbidden

Chapter 1 - A Vertical Velocity of Zero

Upon a rooftop terrace floored in rough-hewn stone, atop a high tower which no one looking at the Hogwarts castle from the outside could see, Harry stood facing Hermione across crossed wands.

"Can you at least not say anything about 'until death takes me', because did I mention I have the Philosopher's Stone now? Or anything about 'the end of the world and its magic'?" He raked his free hand through his hair and gave a small, strained smile. "I'm a lot more nervous around phrases like that than I used to be."

"Well, maybe after this you'll be less nervous," said Hermione. "I do choose this, now." A sharp-edged gust of wind caught her hair, and her chestnut curls billowed around her shoulders. She seemed somehow older, as though an undercurrent of formidable power were bubbling just beneath the surface of her tranquil demeanor.

Harry noticed that his field of vision seemed to have expanded, and that the color saturation of the world had noticeably intensified.

Hermione spoke again, her voice level and precise. "Upon my life and magic I swear friendship to Harry Potter, to help him and trust in him, to stand with him and stand by him, and to sometimes go where he can't go, till the day that death takes me for real - if it ever really does, I mean." She was silent for a moment, then smiled. "And if the world or its magic ends, we'll deal with that together."

A huge grin lit Harry's face. Hermione had been right, he felt much less nervous now about the prophesied destruction of -

If this were a story, that would have been The End.

Harry blinked.

What? Why did I just think that? He frowned at the strange little thought, annoyed that his brain had interrupted itself with such an irrational non sequitur. I mean, I guess it does fit the pattern, in the sense of conforming to standard literary convention...

Far below in the Hogwarts grounds, a large grove of trees swayed in the crisp morning air, drawing Harry's attention back to reality. Spring was rolling into summer in the Scottish Highlands; the wind was light and cool, the sun bright and warm, and Hermione's Magical Aura of Innocence and Purity™ was filling him with a sense of calm peace. It was all too easy to disregard a vague apprehension and shove it to the back of his mind.

He felt much less nervous now about the prophesied destruction of the world at his own hands.

Harry turned his head to gaze out towards the horizon. Thoughts of the future pervaded his mind - Hermione, the Philosopher's Stone, all of the revolutionary things the three of them were going to accomplish together - and he couldn't help but smile again. He thought of the stars, and of prophecies thereof, and he thought of Dumbledore's posthumously delivered letter.

During the First Wizarding War, there came a time when I realized that Voldemort was winning, that he would soon hold All within his hands. In that extremity, I went into the Department of Mysteries and I invoked a password which had never been spoken in the history of the Line of Merlin Unbroken, did a thing forbidden and yet not utterly forbidden. I listened to every prophecy that had ever been recorded, and I learned that my troubles were far worse than Voldemort.

From certain seers and diviners have come an increasing chorus of foretellings that this world is doomed to destruction. And you, Harry James Potter-Evans-Verres, are one of those foretold to destroy it. By all rights, I should have ended your line of possibility, stopped you from ever being born, as I did my best to end all the other possibilities I discovered on that day of terrible awakening. Yet in your case, Harry, and in your case alone, the prophecies of your apocalypse have loopholes, though those loopholes be ever so slight. Always 'he will end the world', not 'he will end life'. Even when it was said that you would tear apart the very stars in heaven, it was not said that you would tear apart the people. And so, it being clear that this world is not meant to last, I have gambled literally everything upon you, Harry James Potter-Evans-Verres.

There were no prophecies regarding how the world might be saved, so I found the prophecies that offered loopholes in the destruction, and I brought about the strange and complex conditions for those prophecies to come to pass. I ensured that Voldemort discovered a certain one of those prophecies, and so (even as I had feared) condemned your parents to death and made you what you are.

Harry shuddered involuntarily. There were certainly timelines somewhere out there in the multiverse where the only point of divergence from his own reality had been a single misstep in Dumbledore's plan. How many possible points of divergence still lay ahead? And how many neighboring timelines were there where Harry had already been - or might yet still be - responsible for the extinction of the entire human race?

But Fate had conspired to have him bound by an Unbreakable Vow to take no action that might cause the world's destruction, and to have Hermione brought back from the dead and bound by oath to keep him in check. If anyone was going to bring Dumbledore's plans to fruition, it was going to be some version of the two of them. Hermione having her own horcrux (and having been imbued during the resurrection ritual with select attributes from both a unicorn and a troll) made it exceedingly unlikely that she'd be dying again any time soon, and the fact that the Philosopher's Stone made Tranfigurations permanent was practically cheating.

Well, just having the Stone isn't enough to go declaring an Instant Global Victory Condition, but it'll still be pretty damn useful. We're going to literally found a New Atlantis. Harry mentally capitalized both words, and snickered to himself. And then... And then we're deliberately going to 'end the world'. A complete overhaul of the fundamentals of civilization, a unification of the human race into a global society of immortal wizards, and then we leave Old Earth behind and start spreading out across the cosmos... He paused; the absurd magnitude of the Baconian project should have felt beyond the realm of possibility, yet every step along the path now seemed tangibly imminent within his own lifetime. And eventually... We're going to Optimize Literally Everything. I still can't believe this is actually happening...

Tears had begun welling up in Harry's eyes, and he quickly lifted a sleeve to brush them away. It was an immeasurable tragedy that The Future™ couldn't be brought about instantaneously, that some who yet lived would still die in the darkness-before-dawn, here at the beginning of all things before intelligent life had fully come into its power. The Stone could make one Transfiguration permanent every three minutes and fifty-four seconds. Even if it only took a single second, there were still six billion people in the world, and six billion seconds was almost two hundred years. And the Unbreakable Vow which Voldemort had forced Harry to make prevented him from immediately tearing down the Statute of Secrecy and extending the Stone's healing to the Muggle world, so it would be a while - years, possibly - before they found a way to truly save everyone; but if immediately granting immortality and the use of magic to the entire human race would result in extinction, then it would have to wait.

Harry turned back again to face Hermione. She was gazing past him at the horizon with an impassive face, and he realized it had probably been a minute or two since she'd finished her oath and he had yet to say a word. "Um, you know, I think future historians might mark this year as the epoch of a new era. Like, we might literally be in the future's 'Year One' right now. That's kinda crazy, isn't it?"

The young witch didn't turn her head. "That must have been quite the internal monologue, Mr. Potter. Almost four minutes without acknowledging your surroundings. Seems a bit excessive, even for you."

Harry froze where he stood.

A roguish grin crossed Hermione's face, and she giggled.

After a moment, Harry chuckled.

Then both were laughing, and all was right with the world.

"Thank you, Hermione," Harry said. "Sorry, I really should've said that immediately. I guess I sort of, um, distracted myself." He smiled sheepishly. "But you really did say just exactly what I needed to hear. You were right, I feel much less nervous now."

The smile left Hermione's face. "I should hope so." Her voice grew a bit cold, took on a sardonic tinge, and her aura drew back and diminished until it faded away entirely. "I specifically wrote that oath in order to elicit a profusion of happiness and hope within your mind, to bring you to an emotional apogée. There's nowhere left to go from here but down."

A chill went down Harry's spine then.

An intimation of something dreadful approaching.

A creeping terror.

An unmistakably familiar sense of doom.

Harry's eyes widened in sudden horror, then his face contorted into a snarl. "Voldemort."

Hermione's lips twisted into a discordantly mirthless grin. "Indeed."

"How dare you use her for -"

"Not an unreasonable assumption," Hermione's voice interrupted, "but no, I did not resurrect her to use as a host. In fact, I never resurrected her in the first place."

"What? But -"

"Your subjective experience of the past 36 hours has, in reality, taken place over the course of approximately 15 minutes." Hermione's hand raised in a broad sweeping gesture, out towards the hills and lakes and trees far below "This is naught but an exceptionally intricate False Memory Charm."

Harry was staring at the form of Hermione as though she'd just turned into a cat. "That can't be true, it takes just as long to create a False Memory Charm as it takes to create a true memory! And I was making choices and problem-solving! I was thinking! I am thinking, right now! There's no way you could've faked my thought-processes so accurately that I'd mistake your fabrications for my own reactions! And, and how could you possibly fake entire conversations? All those people were responding to what I said, and their responses were specific and coherent! That's just absurd! 'Exceptionally intricate' doesn't cut it as an explanation! You'd have to be a -" Harry stopped abruptly, and his heart wrenched as his mind brought forth a memory from what felt like another lifetime.

"I'm well aware of exactly how proficient I am at mental magics, Mr. Potter." A satisfied smile flickered for a moment over Hermione's face, but it died away swiftly. "This is, in all likelihood, the most intricate False Memory Charm to have ever been crafted by any wizard, living or dead. I purposefully orchestrated my stay in the Hogwarts infirmary to ensure that I would have time enough to complete it. The script called for multiple branching points to account for your most likely reactions, though I ensured that your options would be constrained enough to prevent you from taking us off the rails and breaking the illusion. Your conversations were credible because they were actual conversations; I maintained a connection to your mind through Legilimency and performed every role myself. There was a hard limit on how much I could accelerate your cognition, but as a disembodied spirit I was under no such limitations and could freely accelerate to the point that you appeared to be at a standstill, allowing me to perform multiple roles simultaneously. Your thoughts have been your own because the memory is being written onto your brain at precisely the same speed as your mind is operating." Hermione's head turned east towards the sun. "And if I've timed it right, your mind should begin slowing back down right about now."

The sun began to rise noticeably faster.

Wisps of cloud formed and dissipated at an alarming rate.

All the color drained out of the world, and Harry was plunged into darkness.