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The magic of souls is a mysterious thing. Most soul magic is banned, and with good reason, because soul magic is almost always non-reliable. The only exception to that rule is unintentional soul bonds, also known as ‘A Marriage Made In Heaven’. Those who try to force a soul bond come away with massive amounts of unintended consequences, such as when a High Elf tried to force a human witch into one.
House Elves were quite useful nowadays, but almost everyone, including them, had forgotten their true origins. But this story is not about soul bonds, unintentional or otherwise. It is about horcruxes. Horcruxes, depending on who you ask, are the product of enormous study in either Ancient China or Ancient Egypt. By using a ritual on a prepared object, and then murdering someone, you can store a piece of your soul in the object, thereby gaining technical immortality.
So why aren’t there more Ancient Chinese and Ancient Egyptian Dark Lords running around causing havoc? Because, like other soul magic, they very rarely work as intended. One or two of them are hanging around as wraiths, but most are stuck in the horcrux that they made (which was most unfortunate for one Dark Lord who had made a rock his horcrux and then left it in a random field, never to be found again).
Voldemort, surprisingly, or perhaps unsurprisingly, was the first person in history to make more than one horcrux. No one else in history had wanted to mess with soul magic more than once. And as no one had ever before made more than one, no one knew the rules for multiple horcruxes.
So Voldemort had no idea that horcruxes, or rather the bits of soul inside them, shouldn’t ever ever touch. And that Voldemort himself, in wraith form, shouldn’t touch the soul bits either. Not that knowing this would have helped. After all, Voldemort had no idea that the ritual he had cast on Gryffindor’s Shield had been successful. The Shield didn’t have a soul piece in it, because his soul, so damaged from the creation of the other horcruxes, instead latched on to the nearest object, which was Harry’s scar.
The scar which Wraith Voldemort had forcefully traveled through after failing to gain the Philosopher’s Stone. Feeling something trying to attach itself to him, Voldemort batted it away and continued on his way, completely unaware of the now partially dislodged soul bit of himself that was now no longer stuck in Harry’s mind.
Souls, themselves, are infinite. However, a fraction of infinity is still smaller than an entire infinity. So when the sliver of Voldemort’s soul in Harry’s scar met the entirety of Harry’s soul in all of Harry, Harry’s soul immediately latched on and began trying to absorb it.
No one, not Voldemort, not Harry, not Dumbledore, not even Merlin and Morgana themselves, would have been able to predict what happened next.
That is one ugly baby. Am I dreaming? Harry thought. The misshapen baby began to cry and just as Harry managed to touch it, he woke up.
“I guess I was dreaming,” Harry muttered.
“Good Evening Mr. Potter,” Dumbledore said cheerfully.
“Headmaster?” Harry asked. “The Stone! You’ve got to stop Quirrell! He’s going to get the Stone!”