Some years ago, a fan asked J. K. Rowling on her twitter page what would have happened if the fragment of Tom Riddle's soul succeeded in escaping the dairy, to which she replied that the "present day" Voldemort would've become significantly more powerful. Now - I do have a theory on this, but I'm curious to hear yours?

 

The way I see it, there are two possibilities.

Possibility Number One - Riddle leaves Hogwarts and locates Voldemort. With an additional ally, Voldemort becomes "significantly more powerful". This is the possibility that I find most likely.

Possibility Number Two - Voldemort is pulled across the world into the Chamber, regains his body, while Riddle goes back into the Diary. I find this unlikely, it doesn't seem like that's how Horcruxes work.

Horcruxes only do one thing: they prevent you from dying. There is no "Horcruxes combining with the main soul piece". Tom Riddle from the Diary is a Horcrux, regardless if he has a body or not.

 

The funniest theroy would be the two of them both staying separate and fighting in a free for all duel. After that surely Harry would just dust the other. (This is clearly forgetting the other horcruxes)

 

I mean, there is a small inkling chance that Voldemort combines back with his soul piece just to become more powerful. But according to the fandom wiki, it is extremely painful.

I feel like Voldemort wouldn't want to go through all that pain, and his soul is probably so deformed that he wouldn't be able to feel remorse for anything he's done.

That is the first theory, although very unlikely. The second option is just what @Andrela said. It's just that diary Tom Riddle would join Voldemort and they would become a combined force of power. But, considering that diary Tom Riddle has half of Voldemort's soul in him, he would most likely be more powerful than "present" Voldemort (considering that Voldemort only has a 7th of his soul left). In this way, what @Dano1o says is probably the most likely outcome.

 

That's assuming the Horcrux takes half the soul; in canon, it's only said that the soul is split, so any suggestions that the original Horcrux has half the soul of the original, pre-Horcrux Voldemort is only a theory. Also, there's no evidence that I can remember saying that magical strength comes from the soul, so it's likely present-Voldemort would be more powerful in a body than his younger counterpart.

 

The diary horcrux assumes human form, with the memories of a teenage Tom Riddle. He seeks out the master soul and does a much better job than Pettigrew at resurrecting the real Voldemort, which happens a year earlier than canon. Since Harry is dead, no need to use his blood, but horcrux Riddle can harvest blood from Harry's corpse anyway.

War happens, Voldemort wins. Probably puts Diary Riddle back in the book. Doesn't merge with him because the only way to do that is to feel remorse.

Assorted rationales:

- JKR has referred to Voldemort as the master soul in interview.

- The fact that horcruxes are not equivalent to the master soul is shown by the fact that the horcruxes only anchor the master soul. Horcruxes don't anchor each other to life, nor does the master soul act as an anchor for the horcruxes. Horcruxes are fragments of the soul that have been splintered off from it, not independently existing beings.

- Further, Voldemort's ability to control the Nagini horcrux demonstrates that the master soul has the ability to command the others. Dumbledore states in HBP that Voldemort's control of Nagini is greater than you would expect, even from a parselmouth, and that this was evidence of Nagini being a horcrux, telling us that Dumbledore was of the belief that the master soul can control the horcruxes.

- It's important to fully appreciate the significance of what a soul is: a person's identity, their very being. Diary Riddle, being a part of the same soul, would not consider himself a separate entity from Voldemort. They would consider themselves the same person - which indeed we see in the interaction with Harry in the Chamber. A horcrux isn't just a back-up of a person's memories. It is them.

 

I do see what you're saying Taure. However I do think that there would be some conflict with the two souls. I just can't see a megalomaniac like Tom Riddle not fighting with himself for domination. I do agree that the master soul would overcome the diary Riddle and either put him back or kill him all together.

 

It's as Taure says. Horcrux!Riddle is one and the same as Voldemort, with Voldemort being the master. Think of the horcruxes as fingers, if you will, and Voldemort as the hand. The Horcruxes are in all essence indistinct from the master soul, and thus share the same ideals and motivations. There'd be no fighting, because there can be no conflict when all of you wants precisely the same thing.

 

Personally, I think that if Ginny died and the fragment of Voldemort's soul materialized fully from the book, I think that the consequence of it being rendered stronger than the one hiding in Albania would render that a master soul.

A younger, more agile and healthier Dark Lord who is not only (arguably) more powerful, but also equipped with the shrewd and calculated mind of his younger years before his mind grew unstable due to the Horcrux creations... A very scary thought. A more calm and calculated Voldemort would possibly mean that he would not do the same mistakes and considered more carefully before taking action. He might very well have acted a bit less predictable and been much harder to beat or outsmart.