This is a one-shot, what-if type story. Harry sees The Butterfly Effect, and some very strange things happen.
DISCLAIMER: I do not own Harry Potter, nor do I own the Young Wizards series.
Harry Potter himself, the Man-Who-Lived-To-Be-Hyphenated, Boy-Who-Lived, and other such ridiculous titles, sat in Potter Manor and contemplated his very long life. He had defeated The Dark Dimwit by casting nothing more than a simple Disarming Charm. It was rather a let-down, really.
His so-called 'victory' came too late. Thousands of people had died, including many he held most dear.
For one hundred and twenty-three years, he had sought a means to return those that had been lost to life. Upon the final death of He-Who-Flees-From-Death-Like-A-Cowardly-Imbecile, the full force of his power was unleashed. The fragment of Tom Riddle locked in his scar had fed upon his magic in an attempt to grow stronger. The veritable ocean of magic within Harry had flooded his every molecule, changing his DNA on a fundamental level.
The secret to immortality was not some arcane ritual, nor spell, nor potion. It was simply to reprogram the cells in his body to divide with perfection every time. Death, you see, happens because the cells divide imperfectly, more and more over time, resulting in aging.
Harry James Potter, age one hundred fifty-eight, looked just as he did the instant Tom Marvolo Riddle started singing with the Choir Invisible, so to speak.
Harry James Potter, age one hundred fifty-eight, had stumbled across the solution to saving everyone the Serpentine Simpleton had caused the death of whilst watching old movies.
The Butterfly Effect, to be precise.
Whilst traveling physically into the past more than a few hours was impossible, there was nothing to say he couldn't send his mind and magic back. Harry stood and walked into the runic circle he had finally perfected to send himself into his own body in the past, after he performed one last check on the Paradox Buffer that had cost him the most time in devising.
Taking a deep breath, he touched the keystone that would activate the array, and pushed just a drop of his magic into it.
The universe sneezed.
Harry woke somewhere small, warm, and comforting. As he wondered where he had ended up, a bright light appeared, and he was no longer comfortable. His exhausted parents tiredly celebrated his birth. Quietly.
Several hours later, Harry gathered in his enormous magic, and all the ambient magic he could reach, and, in a language older than Creation, reminded the universe at large of the Law of Spiritual Unity, as applies to one Thomas Marvolo Riddle.
The surroundings seemed to lean in to hear him better as he mentally intoned in the Speech, "Hey, universe! Souls can't really be split, you know. It's just not possible. There's even a fancy Law of the Universe about it and everything. So that Tom Riddle fellow, he really should be dead, you see. Those fancy soul anchors of his really can't contain pieces of a soul that can't under any circumstances be split..."
The universe had a seizure. Just a small one though.
Screams echoed, and black wisps of smoke streaked from five different places in England, and slammed into a figure sitting upon a custom-made black throne.
Voldemort, whole in spirit once again, thought 'wait a tick,' right before suffering a massive heart attack from the shock of having his soul forcibly reformed.
10 year old Harry James Potter heads down the stairs of Potter Manor, being enticed onward by the smell of his mother's cooking. As he greeted his father, he looked, as he did every morning, at the newspaper cutting stuck to the charmed ice-box:
"DARK LORD DIES"
July 31st, 1981
Today, Thomas Marvolo Riddle, 81, was found dead by his Marked and very confused followers.
Said Mr. Lucius Malfoy as he was being led out of the court-room to his permanent new home in Azkaban,
"Who would have ever thought the most powerful Dark Lord in centuries would be done in by a heart attack whilst on the shitter?"