The nature and concepts of Horcruxes were so terrible that they were kept secret from most of the wizarding world, and very few people were ever made aware of their existence.
Very few books, even those revolving around the Dark Arts, would mention Horcruxes even in the slightest detail.
Magick Moste Evile, a book that contained much information about the most advanced Dark Arts, barely skimmed over the subject of Horcruxes, stating them to be a subject so evil that it should never be spoken of, showing that even many Dark wizards and witches were terribly horrified by them. In fact, only one book was known to provide specific, detailed instructions on the creation, nature, and consequences of a Horcrux.
Secrets of the Darkest Art.
So if the knowledge for making a Horcrux was so scarce, how little would the knowledge be to reverse the effect of a Horcrux and merge the souls back?
Based on Voldemort's memory, he never came across such findings. There was only one theory about it in the book Secrets of the Darkest Art.
According to Secrets of the Darkest Art, the only known way to truly reunite the pieces of soul contained inside one or more Horcruxes was for the creator to feel genuine regret and remorse for what they had done to make them.
The process of uniting broken pieces of a soul is said to have been extremely painful, and even potentially fatal, to those who did so.
Alas, that was not a possible solution for Voldemort, as he never felt an ounce of regret for killing someone.
"What should I do, what should I do?" Voldemort pondered.
In his search for a solution, Voldemort picked up the two Horcruxes on the table and began looking at them. A few minutes went by, and still, he was without any clue. Just then, his attention was caught by the Marvolo Gaunt ring.
"That's it!" A sliver of insight pierced through the darkness of his thoughts.
The Mervelo Gaunt ring wasn't just a Horcrux. It also held one of the Deathly Hallows. The Resurrection Stone, a stone that could summon the spirits of the dead.
"The stone is powerful enough to connect to the soul of the dead. Compared to that, melding the issue of my soul should be much easier. I will just need to find a way to use the Resurrection stone for my specific use." Voldemort's mind raced with possibilities as he considered the potential of the Resurrection Stone.
Voldemort quickly slid the Marvolo Gaunt ring onto his finger as he looked at the stone for a moment. The Resurrection Stone had a vertical line representing the Elder Wand. The triangle and circle represent the Cloak and the stone.
Then Voldemort closed his eyes and focused his mind on the stone and then on the ring and the Slytherin's Locket. More precisely, focusing on the soul inside the ring and the locket.
As soon as he did that, Voldemort heard a slight movement around him.
He opened his eyes and looked in front of him. There were two entities in the room.
They were neither ghosts nor truly flesh; he could see that. Less substantial than living bodies, but much more than ghosts, they moved toward him.
The first one coming from the ring was a boy of about sixteen with jet-black hair. A silver Prefect's badge was on his chest as the boy was wearing the Slytherin robe. The boy was no other than Voldemort when he was still at Hogwarts.
The second soul that came from the locket had a more mature look compared to the sixteen-year-old Tom Riddle. Instead of the green Slytherin robe, he was dressed in a simple black suit.
Voldemort looked at his own two Horcruxes and smiled. The two souls, each representing different phases of his life, smiled back at him.
He understood now why they looked different. The soul within the Marvolo Gaunt ring depicted him as a sixteen-year-old boy, the age he was when he made that particular Horcrux during a holiday from Hogwarts. It was at that time that he discovered his father was Tom Riddle Senior, a Muggle. Deciding to take revenge against his Muggle father for abandoning his mother, Voldemort murdered his Muggle father, grandfather, and grandmother with the Killing Curse.
The other soul, emerging from Slytherin's Locket, reflected a more mature version of himself. This soul embodied the time when he was working at Borgin and Burkes, where he murdered Hepzibah Smith, a wealthy but elderly descendant of Helga Hufflepuff, and then took possession of the Slytherin Locket and also the Hufflepuff Cup.
His first solution had proven successful. He was indeed able to communicate with the souls contained within the Horcruxes using the Resurrection Stone.
Then Voldemort turned towards the third Horcrux in the room.
As Voldemort gazed at the coiled snake in the corner of the room, his mind filled with anticipation.
He called out, "Nagini."
The snake raised its head, its tongue flickering as it sensed its master's call. Slowly, Nagini began slithering toward Voldemort, its eyes fixed on him.
Having successfully tested on the two non-living objects, Voldemort rested his hand on Nagini's head, feeling the scales beneath his touch. Closing his eyes, he focused his mind once more on the power of the Resurrection Stone, channeling its energy toward the serpent.
As the magic surged through him, a third soul materialized before him, distinct from the other two. It had the shape of a crouched human child. It was hairless and scaly-looking, a dark, raw, reddish black. Its arms and legs were thin and feeble, and its face was flat and snake-like, with gleaming red eyes.
"Those were some unpleasant times." Voldemort had a disgusting expression on his face as he looked at himself in such a state.
In order to gain a body after he was killed by his own killer curse, Voldemort decided to use the Rudimentary body potion. Two of its main ingredients were unicorn blood and Nagini's snake venom. In order to keep himself in a body, Voldemort had to continuously drink the potion every few hours in order to maintain the little health that they had regained. It was during that time that Voldemort made Nagini his last Horcrux.