This story is based on the idea of not all of Riddle's horcruxes being found, but Harry still being victorious. (See "Mage Harry" by kb0 for one example.) Of course, if the horcurxes still existed, Voldemort could still return someday. However, I found myself wondering what might happen if he were unable to do so for some reason.
This story is based on what I was able to research about what scientists say about the far-future fate of the planet Earth. I am not a scientist, and any scientific errors on my part are the result of that fact.
This story is told from Voldemort's perspective.
Harry Potter and all related characters, et cetera belong to JK Rowling. Special thanks to my wife, Sage Defender for her suggestions. Go read her stuff!
The Uncaring Sun
Trapped in a hell of his own making, the shade of Tom Riddle wandered the ruin of the planet Earth. Eerie red light bathed the barren landscape. Uncounted ages had passed since the Potter brat had eventually died of extreme old age, leaving behind a multitude of descendants. That progeny had thrived and prospered through the intervening millennia. Eventually, humanity had taken to the stars, Potter's cursed offspring among them. Finally, the heirs of humanity had been forced to finally abandon the Earth when the sun began its inexorable expansion into a red giant.
As the refugees left, they took with them many of the artifacts of the Earth-that-had-been. However, they did not take everything. Among the many items that remained were a handful of carefully hidden treasures - the horcruxes that still bound Tom Riddle's shade to the Mortal Plane.
As the earth became increasingly inhospitable, the pitiful life forms that had been left behind by those who fled eventually perished. The remnant of the man who called himself Lord Voldemort had no more animals to possess. The increasingly red sun grew larger in the sky across the countless ages and the sea inevitably boiled, creating even more eerie landscape for Riddle's shade to examine in excruciating detail.
Due to the instability created by the fracturing of his soul, Voldemort had not been quite sane even when he still had inhabited his own body. Nevertheless, that insanity paled in comparison to what he experienced in the countless millennia of solitude on a dead planet.
Even while the Earth had still teemed with life, he had been forgotten, a mere footnote in the history books. Although Dumbledore, Potter, and their allies had been unable to locate all of his horcruxes, what they had done to him had been even worse. Every one of his Death Eaters who had not been killed in the fighting had been sent through the Veil of Death. As a result, once Potter had eventually died, no one living knew that Lord Voldemort had journeyed farther on the path to immortality than anyone before him, much less that he had actually made horcruxes. No one would be able to seek his horcruxes for his resurrection and they were too well hidden to be accidentally discovered. Likewise, there would be no Wormtail to know that his disembodied form still existed and seek it.
In the years following his defeat, Voldemort had tried to find other willing hosts like Quirrell, but he had been unable to find one. Dumbledore and Potter had somehow found a way to prevent him from interacting with Wizardkind. He could not forcibly take possession of even the weakest-willed wizard. Neither would a potentially willing host have been able to sense his presence and invite him in. As a result, he had been forced to endure the years and the ages suffering in his disembodied state. Voldemort did not know whether they had found the technique or created it, nor was he interested in learning about their method. All that he knew was the agony of existing as nothing but an intangible wraith. For a time, he could at least take some pleasure in the fear that his name still inspired in the Wizarding populace, but that soon faded. Eventually, he was forgotten as the centuries and millennia passed and other would-be Dark Lords rose and fell.
From time to time, the remainder of what had been Tom Riddle encountered others like him, the sad remnants of shattered souls. Like him, they had all tried to cheat death and fate by unholy means. Like him, they were all abandoned in their disembodied state: friendless, followerless, alone. They could give him no comfort. He could give none to them, not that he would have ever done so.
Billions of years passed and the sun grew ever larger in the daytime sky. The seas inevitably boiled and much of the atmosphere was stripped away by the solar wind. Still, the shade of Tom Riddle was bound to the hellish landscape, trapped in the nightmare of an existence he had wrought for himself. His remaining horcruxes were still quite safe from the relentless radiation in their underground caches.
By this point, the being who had once been Tom Riddle would no longer have been recognizable, even to himself. He no longer recognized the passage of time, only the never ending agony that was his sole reality. The phantom of a spirit no longer thought of himself as a being, nor was he any longer aware of his own existence. He did not remember his corporeal life, nor his servants, nor his many adversaries. The capacity for understanding and coherent thought had been stripped from him like the atmosphere had been from his now dead home. Instead, he had been reduced to the madness of bestial survival by the passage of time and the inability to die.
Eventually, the very earth itself began to melt under the incessant onslaught of the engorged sun. As the ground softened and then flowed, the objects that contained the fragments of Tom Riddle's shattered soul were one by one destroyed. Riddle's elation at his new-found freedom from his hellish existence was short-lived, however, as he discovered that there was indeed an actual place called Hell.
The uncaring sun continued its expansion and the innermost planets finally merged with its engorged form, their component atoms lost in its plasma.
Eons later, an unremarkable white dwarf star burned in the centre of a solar system vastly different from the one it had once overseen in its younger days. The star that had once given warmth and life to its planets had brought them death. Its assault completed, the red giant had cooled and shrunken into the next phase of its life-cycle. The descendants of the beings that had long ago fled the dying planet Earth had long since forgotten their solar system of origin. The unholy objects that had been made by a series of dark wizards had long since been obliterated.
I did find some conflicting opinions regarding whether or not the sun would engulf the Earth or not, due to a number of factors. As I said, I'm not a scientist.
I have this headcanon that horcruxes can be destroyed by radiation. I didn't use it here, but I did reference it.
The cover image was created using an image licensed using a Creative Commons license. Therefore, the cover image is also covered under such a license. The original may be found on Wikipedia in the article about The Sun. The file name is Sun(underscore)Red(underscore)
Sfjoellen points out that one of the horcrux-bound spirits present could be that of Herpo the Foul (the wizard who invented the horcrux). Since Voldemort wouldn't care about anyone but himself, I chose not to give identities to the other shades mentioned in this story, but that has been added to my headcanon.